There is no in-game reason to colonize; it ties in to no other game loops; it enhances no other gameplay; it provides no actual benefit. Colonization doesn't even benefit further colonization! Like many aspects of this game, there is a ton of potential in the concept of colonization but the implementation is essentially busy work.It's just another thing to do in the game. Like every other feature of the game, it's not for everybody.
Unfortunately, they were quite clear that Colonization is "feature complete", despite the "beta" label they're slapping on it, so I don't have confidence that there are any changes on the horizon that will better integrate colonization into the rest of the game. I would love to be proven wrong on that, though.