We have information from the Bhagavad-gītā that the Lord appears at particular intervals to adjust a time-worn spiritual culture. Lord Śrī Kṛṣṇa appeared at the end of Dvāpara-yuga to regenerate the spiritual culture of human society and also to manifest His transcendental pastimes. Viṣṇu is the authorized Lord who maintains the created cosmos, and He is also the principal Deity who makes adjustments when there is improper administration in the cosmic creation. But Śrī Kṛṣṇa, being the primeval Lord, appears not in order to make such administrative adjustments but only to exhibit His transcendental pastimes and thus attract the fallen souls back home, back to Godhead.
However, the time for administrative rectification and the time for Lord Śrī Kṛṣṇa’s appearance coincided at the end of the last Dvāpara-yuga. Therefore when Śrī Kṛṣṇa appeared, Viṣṇu, the Lord of maintenance, merged with Him because all the plenary portions and parts of the absolute Personality of Godhead merge with Him during His appearance.