Unless you happen to be playing as the Advent with maxed culture research and the Manifest Dominion artifact and have Desert planets scattered evenly across the galaxy filled purely with Communion Temples, victory by culture is practically impossible.
Culture, aside from dropping alleigance, can also delay or prevent enemies from recovering lost planets, since they cannot recolonize planets that have revolted or are heavily under your culture's influence (until your culture centres are destroyed, that is). Its also useful to give your fleets bonuses at locations where your culture has reached and to deny the enemy of their bonuses at territory under your culture.
