What reasons do you have for colonization?

In my setting, massive amounts of colony ships are being sent out to settle on planets, but i'm having trouble justifying why such a thing would be practical/possible resource-wise. The main ideas I have right now are:

• The colonisation is managed by AI with near-limitless resources
• There are multiple different ages of colonization, rather than one big one.
• Lots of smaller missions are sent out.