When He (Krsna) is so requested by devotees like Brahma, He descends on the earth for the welfare of the entire universe. This is stated in Bhagavad-gita (4.8): paritranaya sadhunam vinasaya ca duskrtam/ dharma-samsthapanarthaya sambhavami yuge yuge

Expressions researched:
"when He is so requested by devotees like Brahmā, He descends on the earth for the welfare of the entire universe. This is stated in Bhagavad-gītā (4.8): paritrāṇāya sādhūnāṁ vināśāya ca duṣkṛtām/ dharma-saṁsthāpanārthāya sambhavāmi yuge yuge"

SB Canto 3

The Lord has no business in this world, but when He is so requested by devotees like Brahmā, He descends on the earth for the welfare of the entire universe.

SB 3.2.25, Translation and Purport:

The Personality of Godhead, Lord Śrī Kṛṣṇa, being prayed to by Brahmā to bring welfare to the earth, was begotten by Vasudeva in the womb of his wife Devakī in the prison of the King of Bhoja.

Although there is no difference between the Lord's pastimes of appearance and disappearance, the devotees of the Lord do not generally discuss the subject matter of His disappearance. Vidura inquired indirectly from Uddhava about the incident of the Lord's disappearance by asking him to relate kṛṣṇa-kathā, or topics on the history of Lord Kṛṣṇa. Thus Uddhava began the topics from the very beginning of His appearance as the son of Vasudeva and Devakī in the prison of Kaṁsa, the King of the Bhojas, at Mathurā. The Lord has no business in this world, but when He is so requested by devotees like Brahmā, He descends on the earth for the welfare of the entire universe. This is stated in Bhagavad-gītā (4.8): paritrāṇāya sādhūnāṁ vināśāya ca duṣkṛtām/ dharma-saṁsthāpanārthāya sambhavāmi yuge yuge.