In Krishna-lila, Nityananda Prabhu is Sri Balarama.

Sri Balarama is assisting Sri Krishna in His enjoyment as Master of Lilas.

Everything in the spiritual world which assists in or enhances Krishna´s lila is an expansion of Sri Balarama.

Krishna Das Kaviraja Goswami says:
„Nityananda is Balarama, Krishna´s elder brother. Just as Sri Krishna of Vrindavana appeared as Sri Caitanya of Navadvipa, Balarama of Vrindavana appeared as Nityananda of Navadvipa.“

Sri Krishna is the Supreme Person, Bhagavan Himself and Sri Balarama is His first manifestation in a different body to assist Krishna in His lila.
Sri Krishna is Sevya, the One who is served. Balarama is Sevaka, the One who serves.

So Nityananda is the perfect servant, in all respects.

Just as Balarama serves Krishna in numerous ways, Nityananda serves Sri Caitanya.

Without the mercy of Nityananda, it is not possible to attain the highest goal, pure love (prema).

“The love for Krishna in the mood of the gopis which is described in the Bhagavata is attained in the world through Nityananda.”
Caitanya Bhagavata, 3.5.303

Srila Narottama das Thakur sings:

heno nitāi bine bhāi, rādhā-kṛṣṇa pāite nāi

“Unless one takes shelter under the shade of the lotus feet of Lord Nityananda, it will be IMPOSSIBLE to attain Radha-Krsna.”

Why is it impossible?

Nityananda Prabhu is giving us Gaura-prema, love for Gauranga. For us, Gauranga is primarily in the bhava of Srimati Radhika, so giving away Gaura-prema means distributing love for Srimati Radhika, meaning giving away prema in manjari-bhava, serving the Divine Couple as a beloved maidservant.
Sriman Mahaprabhu appeared to distribute EXACTLY this kind of love.

anarpita-carim cirat karunayavatirnah kalau
samarpayitum unnatojjvala-rasam sva-bhakti-sriyam
harih purata-sundara-dyuti-kadamba-sandipitah
sada hridaya-kandare sphuratu vah saci-nandanah

May the Supreme Lord who is known as the son of Srimati Saci-devi be transcendentally situated in the innermost chambers of your heart. Resplendent with the radiance of molten gold, He has appeared in the Age of Kali by His causeless mercy to bestow what no incarnation has ever offered before: the most sublime and radiant mellow of devotional service, the mellow of conjugal love.

unnatojjvala-rasam sva-bhakti-sriyam – this is NOT general bhakti, but the HIGHEST bhakti. It is the love in the hearts of the Vraja-sundaris, the maidservants of Srimati Radhika. Sriman Mahaprabhu came to give us nothing more and nothing less.

So without the mercy of Sri Nityananda Prabhu, we can´t attain the mercy and love for Sriman Mahaprabhu…and without the mercy of Sriman Mahaprabhu we can´t attain the gift He had come to give: RADHA-DASYAM…Ergo, the mercy of Sri Nityananda is very important for attaining the goal of life, Radha-dasyam.
Sriman Mahaprabhu is full of unnatojjvala-prema Himself, and He came to give us this “unnatojjvala rasam sva-bhakti-sriyam” (the wonderful service to Srimati Radhika as Her maidservant).

So although Nityananda is the same as Balarama, there exists an aspect in which Nityananda differs from Him.
Balarama is the eternal brother of Krishna in Krishna´s Vraja-lila, where Krishna enjoyed His prema-lila. So Balarama was not able to distribute prema in this Vraja-lila.
Now, as an eternal „brother“ of Sri Caitanya, Nityananda takes part in a different kind of lila. Mahaprabhu´s lila is described as prema-dana-lila: the lila of giving away prema freely. So here Nityananda was able to distribute prema to everyone without any consideration. In fact, He gave away the love of the gopis of Vraja, the zenith of pure love. Also, Nityananda „excels“ Balarama in the tattva of RASA.
In Charita-sudha we can read:

„Just as Gauranga is the combined form of Radhika and Krishna, Nityananda is the combined incarnation of Balarama AND Ananga Manjari. Therefore Nityananda is the repository of all the different kinds of bhava and rasa, which Balarama AND Ananga Manjari have. Besides, there are in Nityananda some very unique rasas (spiritual emotions) which are very hard to describe.“
(Carita-sudha, p. 167)

Srila Vrindavana Das Thakur tried to give us some idea of them in a verse, which compares the bhava (inner mood) of Nityananda with the bhava of Radhika (Ananga Manjari is VERY VERY dear to Radhika!).
Some of these lines are like that:

„basi bama pase, mridu hase, prananath boli dake
radhara jemana manera vasana temani kariya thake.”

Nitai sits to the left of Gaura, smiling sweetly and calls Him Prananatha (beloved).
He acts with Him as if Radhika would desire to do.

„das vrindavana kare nivedana, dekhite nitai radha“

– Vrindavana Das says that (on such occasions), Nitai looks like Radha (again, Ananga Manjari is very very close to Radhika).“
(Padakalpataru, pp. 366-367)

Also, Vrindavana das Thakur called Nityananda Purna-sakti (the full potency), Seva-vigraha (embodiment of service) and Murti-mana Krishna-prema (the embodiment of Krishna-prema):

nityananda prasade se haya vishnu-bhakti
janiha krishnera nityananda purna shakti
(C.Bhag. Madhya 12.26)

„One can surely get devotional service to Vishnu (Krishna) only by the mercy of Nityananda. Know that Nityananda is a complete potency of the Supreme Lord.“

Nityananda Prabhu is the personification of Caitanya Mahaprabhu’s most magnanimous mercy.

ye bhakti gopikā-gaṇer kabe bhāgavate
nityānanda hoite tāhā pāila jagate

“The love for Krishna in the mood of the gopis which is described in the Bhagavata is attained in the world through Nityananda.”
(Caitanya Bhagavata, 3.5.303)

Mahaprabhu tells Raghava Pandit:
“I tell you confidentially that Nityananda is completely non-different from me.”
(Caitanya Bhagavata, 3.5.100-104)

nityananda namas tubhyam premananda-pradayine
kalau kalmasha-nasaya jahnava-pataye namah
(Sri Stava-kalpa-druma)

“I offer pranama unto Sri Nityananda Prabhu who bestows the bliss of divine love, who eliminates the contamination of the age of Kali, and who is the master of Jahnava-devi.”

caitanyera adi-bhakta nityananda-raya
caitanyera yaso vaise yanhara jihvaya
ahar nisa caitanyera katha prabhu kaya
tan’re bhajile se caitanye bhakti haya
CB Adi 9.217-218

Lord Caitanya’s first and foremost devotee is Nityananda Raya. The glories of Lord Caitanya are always on his tongue. Day and night Lord Nityananda speaks only of Lord Caitanya. Whoever worships Him is a real devotee of Sri Caitanya. Nityananda is the gateway through which all service to Lord Caitanya must pass.

nityananda avadhuta sabate agala
caitanyera dasya-preme ha-ila pagala
CC Adi 6.48

Nityananda, the wandering mendicant, is the foremost of all the servants of Lord Caitanya. He is like the gateway through which all service to Lord Caitanya must pass. He became mad in the ecstasy of service to Lord Caitanya.

preme matta nityananda kripa-avatara
uttama, adhama, kichu na kare vicara
ye age padaye, tare karaye nistara
ataeva nistarila mo-hena duracara
CC Adi 5.208-9

Nityananda is intoxicated by ecstatic love and is an incarnation of mercy, He makes no distinction between the exalted and the fallen. He delivers all those who fall down before Him. Therefore He has delivered such a sinful and fallen person as me.

dui bhai eka-tanu samana-prakasa
nityananda na mane, tomara habe sarva-nasa
ekete visvasa, anye na kara sammana
“ardha-kukkuti-nyaya” tomara pramana
CC Adi 5.175-176

These two brothers (Gaura and Nitai) are like one body; they are identical manifestations. If you do not believe in Lord Nityananda, you will fall down. If you have faith in one, but disrespect the other, your logic is like the logic of accepting half a hen.

sri-caitanya – sei krishna, nityananda – rama
nityananda purna kare caitanyera kama
CC Adi 5.157

Lord Caitanya is the same Lord Krishna, and Lord Nityananda is Lord Balarama.
Lord Nityananda fulfills all of Lord Caitanya’s desires.

jaya jaya nityananda, nityananda-rama
yanhara kripate painu vrindavana-dhama
jaya jaya nityananda, jaya kripa-maya
yanha haite painu rupa-sanatanasraya
yanha haite painu raghunatha-mahasaya
yanha haite painu sri-svarupa-asraya
sanatana-kripaya painu bhaktira siddhanta
sri-rupa-kripaya painu bhakti-rasa-pranta
jaya jaya nityananda-caranaravinda
yanha haite painu sri-radha-govinda
CC Adi. 5.200-204

All glory, all glory to Lord Nityananda Balarama, by whose mercy I have
attained shelter in the transcendental abode of Vrindavana. All glory, all glory to the merciful Lord Nityananda, by whose mercy I have attained shelter of Sri Rupa and Sanatana. By His mercy, I have attained the shelter of the great Sri Raghunatha dasa Gosvami, and by His mercy I have found the refuge of Sri Svarupa Damodara. By the mercy of Sanatana Gosvami I have learned the final conclusions of devotional service, and by the grace of Sri Rupa Gosvami I have tasted the highest nectar of devotional service. All glory, all glory to the lotus feet of Lord Nityananda, by whose mercy I have attained Sri Radha-Govinda.

Some excerpts are taken from O.B.L Kappor´s book “The companions of Sriman Mahaprabhu”