What does a Protectorate do?

[andrewt]

The Senate asked me to obtain Carthage's surrender. For 5,000 denarii and increased goodwill, I said why not. Now that they are a protectorate of mine, what exactly do they do? Do they help me in my wars or anything? One of their towns has so many peasants and town militia and is just a town with 537 population and the other (their capital) has a peasant revolt.

They're most likely useless but I still wnat to know the benefits to me.

edit: I got no stat increase after successfully making them a Protectorate. I asked what would they want in exchange for giving me tribute, they said no and my diplomat got a huge stat increase. Funny.