Colonia

[aln]

What is the purpose of the Colonia build option. I noticed it for the first time while playing earlier on. It has appeared in the Roma build options but is very expensive to just build and see what happens. The description doesn't really give enough information as how to use it. I obviously know the historical uses of colonies (placating veterans, romanisation, trade protection and overpopulation), But in the game is it purely to ease population issues or do I get to place the resulting building in any provence.
A nice touch would be if a colony gave you a unit of veteran principii which could not leave its province, so it would purely be a defensive unit. Pretty sure this is not possible though. I suppose it could be a house rule.