Krishna Das Kaviraj Goaswami explains in Chaitanya Charitamrit –

saṅkarṣaṇaḥ kāraṇa-toya-śāyī garbhoda-śāyī ca payo ’bdhi-śāyī
śeṣaś ca yasyāṁśa-kalāḥ sa nityā-nandākhya-rāmaḥ śaraṇaṁ mamāstu – (CC Adi 1.7)

“May Śrī Nityananda Rāma be the object of my constant remembrance. Saṅkarṣaṇa, Śeṣa Nāga and the Viṣṇus who lie on the Kāraṇa Ocean, Garbha Ocean, and ocean of milk are His plenary portions and the portions of His plenary portions.”

It means in Kaliyuga Shri Krishna incarnated as Chaitanya Mahaprabhu and Balram ji as Nityananda Prabhu. Nityananda Prabhu is non different from Chaitanya Mahaprabhu in all respects except bhaav, love. Shri Gauranga Mahaprabhu is the worshippable Lord who accepts service whereas Shri Nityananda Prabhu is the servant Lord who serves. Thus the Lord Himself demonstrates how one can serve God.

In the spiritual world, Balram ji is the first expansion of Shri Krishna. From Balram ji the first quadruple expands – Sankarshan, Vasudev, Pradyumna, Anirudha. Sankarshan further expands into Mahasankarshan which later expands into a second quadruple – Sankarshan, Vasudev, Pradyumna, Anirudha. This expansion of the Lord as Sankarshan assists in the creation of the spiritual world, which is 3/4th of the Lord’s entire creation. It means that the innumerable number of Vaikutha dhaams are nothing but the expansion of Lord Balram ji to serve the Lord. To create the material world, Balram further expands Himself into three Viṣṇu – Maha Viṣṇu also known as Kāraṇodakaśāyī Viṣṇu, Garbhodakaśāyī Viṣṇu, and Kṣīrodakaśāyī Viṣṇu.

Maha Viṣṇu (Kāraṇodakaśāyī Viṣṇu) – Innumerable material universes come out of His body pores when He exhales. What to talk of this particular universe, we are unable to ascertain the beginning and end of this world. Such infinite universes are coming out from the body pores of Maha Vishnu, one can wonder how immense His form would be.

Garbhodakaśāyī Viṣṇu – In each one of the universes the Lord enters as Garbhodakaśāyī Viṣṇu. From the navel of Garbhodakaśāyī Viṣṇu has sprung the stem of the lotus flower, the birthplace of Brahmā, who is the father of all living beings and the master of all the demigod engineers engaged in the perfect design and working of the universal order.

Kṣīrodakaśāyī Viṣṇu – From Garbhodakaśāyī Viṣṇu there is expansion of the Kṣīrodakaśāyī Viṣṇu, who is the collective Paramātmā of all living beings. Paramātmā resides in all 84 lakh species of living entities as Supersoul. He is called Hari, and from Him, all incarnations within the universe are expanded.

All these three Viṣṇus are expanded from Balram ji aka Nityananda Prabhu!

In 1474, Nityananda Prabhu appeared in the village Ekachakra of Bengal. He was born to a religious Bengali Brahmin, known as Pandit Hadai, and his wife, Padmavati. They are the real parents of Nityananda Prabhu in Goloka Vrindavan. At an early age, He used to re-enact various pastimes of the Lord with His friends. They all were amazed at how many details of the Lord’s pastimes Nityananda Prabhu knew at that age. When they ask Him, He would simply say, these all wereHis pastimes. They would all laugh and no one would believe Him. After a few years, in the company of a Sanyasi, he visited all the pilgrimages, then started living in Vrindavan. While residing there, he got to know that Chaitanya Mahaprabhu had started the sankirtan movement. He came to meet Mahaprabhu and took permission. After that, He started preaching Krishna consciousness.

It is impossible to get the mercy of Shri Chaitanya Mahaprabhu without getting mercy of Shri Nityananda Prabhu. Whatever knowledge of Chaitanya Mahaprabhu’s life we have available today is only due to the mercy of Nityananda Prabhu. One gets the power to glorify and to preach from His Nityananda Prabhu because, in His AnantShesha form, He empowers.

Krishna Das Kaviraj Goswami, the author of Chaitanya Charitamrit, got instructions from Nityananda Prabhu in his dreams. Nityananda Prabhu instructed him to go to Vrindavan and all his desires would be fulfilled. Vrindavan Das Thakur, the author of Chaitanya Bhagwat, was also inspired by Nityananda Prabhu. This concludes that due to the mercy of Nityananda Prabhu only we get to know about Chaitanya Mahaprabhu’s life and pastimes. Therefore, Nityananda Trayodashi is of utmost importance to all devotees who are aspiring to Krishna prema. Nityananda Prabhu is Adiguru, who teaches us how to approach and take shelter of the lotus feet of Godhead. Therefore, to get love of Godhead, to get the mercy of Shri Chaitanya Mahaprabhu, to deliver the fallen souls of Kaliyuga we must have the mercy of Shri Nityananda Prabhu. Thus we should take shelter of His lotus feet and pray for His mercy, because Shri Chaitanya Mahaprabhu can’t deny and has to give shelter to someone if Nityananda Prabhu approves him. We know that Mahaprabhu was about to kill Madhai, but left him alive because Nityananda Prabhu intervened. That’s why Nityananda Trayodashi is of much significance for us.