Coloniae are very nice to have. Historically, if a city starting growing too much, colonies would be established. Greece would do them overseas (Syracuse and Tarentum for example) and Rome would do them in conquered territory. While there isn't any changes to the map and you cannot make new cities, it is an excellent way to slowdown growth and increase law in the city.

If you are at war with Carthage, go ahead and send two small armies to take Sardinia and Corsica. Sicily is another matter. Generally, such large stacks show up there that Carthage can never take Syracuse, but the money you would get from there is always nice.