In Brihadaranyaka Upanishad: Now, these are, verily, the three worlds: the world of men, the world of the Manes and the world of the gods. The world of men can be gained through a son only and by no other rite; the world of the Manes through rites; and the world of the gods through meditation. The world of the gods is the best of the worlds. Therefore they praise meditation.
The world of the Manes means pitris or ancestors and Manes - souls of dead ancestors
And appeasing the Manes is even more important than pleasing the Gods as said in the following verse:
Manu Smriti 3.203. For twice-born men the rite in honour of the manes is more important than the rite in honour of the gods; for the offering to the gods which precedes (the Sraddhas), has been declared to be a means of fortifying (the latter).
In Adition, It is one of 3 runas (debts) we borrow when taking birth as a man - to pitrus, which is repaid by begetting a son doing tarpana/shraddam.