yad-artham adadhad rupam
etan no bhagavan sarvam
yathavad vaktum arhasi
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
What was the purpose for which the Supreme Personality of Godhead accepted the abominable form of a fish, exactly as an ordinary living being accepts different forms under the laws of karma? The form of a fish is certainly condemned and full of terrible pain. O my lord, what was the purpose of this incarnation? Kindly explain this to us, for hearing about the pastimes of the Lord is auspicious for everyone.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Pariksit Maharaja’s question to Sukadeva Gosvami was based on this principle stated by the Lord Himself in Bhagavad-gita (4.7):
yada yada hi dharmasya
glanir bhavati bharata
tadatmanam srjamy aham
“Whenever and wherever there is a decline in religious practice, O descendant of Bharata, and a predominant rise of irreligion — at that time I descend Myself.” The Lord appears in each incarnation to save the world from irreligious principles and especially to protect His devotees (paritranaya sadhunam). Vamanadeva, for example, appeared to save the devotee Bali Maharaja. Similarly, when the Supreme Personality of Godhead accepted the abominable form of a fish, He must have done so to favor some devotee. Pariksit Maharaja was eager to know about the devotee for whom the Supreme Lord accepted this form.