The Wiki probably describes it better than me (this link specifically), but essentially when your colony completes you get a bonus to goods produced by the following formula "native_pops / 20,000". So if your colony had 5,000 native pops it would get a 0.25 bonus to goods produced upon completion of the colony (which is approximately equivalent to one diplo development). However the native assimilation bonus of say 100% would effectively double your goods produced per a native pop upon completion of the colony. That same colony I referred to earlier would now give 0.5 bonus goods produced upon completion. This can be extremely helpful as goods produced increases both your production income in a province and your trade income from the node that province is in. I believe with France you can get that bonus to 175% Native Assimilation, which will be giving you about 0.14 goods produced per 1,000 native pops upon completion which is nearly the equivalent of one extra diplo development per every 1,000 native pops (about 0.75 development more specifically).
