NOTE. The basic principle behind Bhagīratha’s prayer is to distribute Lord Viṣṇu’s mercy upon his forefathers, for Gaṅgā has emanated from His lotus feet.

This principle is elaborated by Śrīla Prabhupāda in his purport to Bhagavad-gītā 1.41: “According to the rules and regulations of fruitive activities, there is a need to offer periodical food and water to the forefathers of the family. This offering is performed by worship of Viṣṇu, because eating the remnants of food offered to Viṣṇu can deliver one from all kinds of sinful actions. Sometimes the forefathers may be suffering from various types of sinful reactions, and sometimes some of them cannot even acquire a gross material body and are forced to remain in subtle bodies as ghosts. Thus, when remnants of prasādam food are offered to forefathers by descendants, the forefathers are released from ghostly or other kinds of miserable life. Such help rendered to forefathers is a family tradition, and those who are not in devotional life are required to perform such rituals.”

Such rituals involve taking shelter of Gaṅgā.

Lord Brahmā, the spiritual master of the universe and a direct disciple of Lord Garbhodakaśāyī Viṣṇu, compassionately wanted to establish such rituals on earth for the benefit of those who are not in devotional life and so appreciated Bhagīratha’s prayer.