They can be useful to trade with, for treaties, or arrange marriages. Furthermore, their cities have some small but inherent advantages such as crystal or mining based on their function. They might have a crystal resource you don't have access too and can trade for.
@Carewolf, most of the army types are dependent on resources and technology. I haven't even really seen a minor race build any units so it wouldn't be too useful to conquer them for their army. I don't know if you can use the Empire's army after conquering their city... probably not because you don't have their techs or resources.
