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. One who is engaged in the devotional life is not required to perform such actions. Simply by performing devotional service, one can deliver hundreds and thousands of forefathers from all kinds of misery. It is stated in the Bhāgavatam:

devarṣi-bhūtāpta-nṛnāṁ pitṝṇāṁ
na kiṅkaro nāyamṛṇī ca rājan
sarvātmanā yaḥ śaraṇaṁ śaraṇyaṁ
gato mukundaṁ parihṛtya kartam

"Anyone who has taken shelter of the lotus feet of Mukunda, the giver of liberation, giving up all kinds of obligation, and has taken to the path in all seriousness, owes neither duties nor obligations to the demigods, sages, general living entities, family members, humankind or forefathers." (Bhāg. 11.5.41) Such obligations are automatically fulfilled by performance of devotional service to the Supreme Personality of Godhead.