Sri-Krishna beautifully reveals in Bhagavad Gita that why he shows his Universal form. Anyway, Some reasons are -
Because Arjun requested Sri-Krsna.
Arjuna says to Sri Krsna -
evam etad yathattha tvam, atmanam paramesvara, drastum icchami te rupam, aisvaram purusottama - Bhagavad Gita 11.3
O greatest of all personalities, O supreme form, though I see here before me Your actual position, I yet wish to see how You have entered into this cosmic manifestation. I want to see that form of Yours.
Very obvious. In the case of Arjun.
Although Krsna has no obligation to show himself or universal form to anyone. It's his greatness that he reveals it to his devotees.
To educate the future Kali-yuga people to stop worshipping the self-made Gurus, upstart Godman/Godwomen/pseudo incarnation etc
Simple acid test Question: is anyone claiming themselves as God/incarnation of Shiva, Vishnu etc? Well, ask them to show Vishva-Rupa aka universal form. Let us see infinite universes inside them. Simple.
Krsna also understands that Arjuna wants to see the universal form to set a criterion, for in the future there would be so many imposters who would pose themselves as incarnations of God.
The people, therefore, should be careful; one who claims to be Krsna should be prepared to show his universal form to confirm his claim to the people.
Be careful. A man can never become a god. That's it.
Krishna is Special.
As per Vaishnava texts, Sri-Krsna has main 6 opulences and other 64 qualities.
And 2nd Quality is "Saulabhya" - easily accessible. That's what set Vishnu/Krishna apart.
We know It's next to impossible for a common ppl like us to approach a PM or president of the country. Ok, leave, not even a Ward member :)
In contrast, Sriman Vishnu, from his single breath billions and trillions of universes comes into existences. He is lord of Infinite Worlds. Still, he is extremely accessible.
Just call his name with love. He will be available next to you to protect you as a loving father anxiously guards his young kid.
For curious ones - main 6 Qualities are
aiswarasya samägrasya viryasya yasasah sriyah, jnäna vairägyayos caiva sannam bhaga itingana - Vishnu Purana 6.5.47
- Knowledge (jnana) - Krsnam Vande Jagat guram... Krsna is the supreme teacher and master of the entire Universe...
- Wealth (aiswarasya) - He is Srinivasa Govinda... Husband of Goddess of Fortune and wealth, Srimati Mahalaksmi Devi... Needs no further explanation :)
- Fame (yasasah) - thousands of volumes transcendental literature, books, Vedas, Upanishad, Vedanta Sutras, Puranas, Brahma Sutras, Ramayana...
- Strength (viryasya) - he lifted Govardhan with the Little finger when he was 7 yrs old kid. Entire Mandara mountain on back in Samudra Manthan Lila.
- Beauty (sriyah) - huh! Lord Bramaji prayers in Bramaha samhita, Kandarpa koti kamaniya vishesha... Krsna is More handsome than millions of Cupid.
- Renunciation (yasasah) - He As great Sanyasi Nara-Narayana, meditates high in the Himalayas, Badrinath. Although he is a great Sanyasi, still he has 16000 wives in Dwarka. Pretty hard to get. Only eyes smeared with a tinge of loving devotion can get these.
Lord Krsna has these opulences and Qualities in totality, Ocean. whereas even great demigods like Indra, Bramaji etc have them infinitesimal, a droplet of the ocean.
To show impersonalist that there is much more.
As per Srimad Bhagavatam 1.3.3 purport, The conception of the Virat-rupa, especially for the impersonalist.
Impersonalists means there is no enjoyment. There is simply light, a life of knowledge. They don't believe in a personal form of god.
gopān vayasyān ākṛṣya, taiḥ saṁsṛjya vijahratuḥ, vādyamāneṣu tūryeṣu,valgantau ruta-nūpurau - Śrimad Bhagavatam 10.44.29
“Krishna and Balarama then called Their young cowherd boyfriends to join Them, and in their company the Lords danced about and sported, Their ankle bells resounding as musical instruments played.”
As said, we too (devotees) can join Krishna and their company in the Spiritual world, Vaikunta. to sing play and dance with supreme personality with the godhead, Krsna, face to face. After leaving this body :)
As impersonalists can't, hence the all-merciful Lord Krsna showed his Virat Rupa.
Out of love, Causeless mercy.
The great personalities who saw the Universal form of Krsna are -
- Great Heavenly sages like Narada Muni
- Sages like Vyasadeva
- The Adityas, Vasus, Sādhyas and other heavenly demigods.
All are devotees. Unanimously.
Lord Sri-Krsna in Bhagavad Gita says -
bhaktya tv ananyaya sakya, aham evam-vidho 'rjuna, jnatum drastum ca tattvena, pravestum ca parantapa - Bhagavad gita 11.54
My dear Arjuna, only by undivided devotional service can I be understood as I am, standing before you, and can thus be seen directly. Only in this way can you enter into the mysteries of My understanding.
Krsna can be understood only by the process of undivided, unflinching devotional service.
He explicitly explains this in this verse so unauthorized commentators, who try to understand Bhagavad-gita by the speculative process, will know that they are simply wasting their time.
Lord Krsna has no obligation to show himself. He is a fully independent lord of the infinite universes. He has none to please. He is ever self-satisfied lord. but he always pleases his dear devotees out of pure love for them.
Hare-Krishna jai Radharamana.