It spawns a basically immobile family member. It gets 100% movement penalty so he cannot move unless he has some traits that increase that such as energetic. There's a fix that makes it so that the penalty drops down as there's a problem with sallying when there's a client ruler in the city. Don't bother trying to fight off rebels with him, he can't move much.

Generally, a client ruler, I would say, isn't worth it. Their upkeep (around 700) is usually higher than what they contribute to the income of the city. They only get past that once they grow a lot older, but by that time you've have lost a lot of potential money already. But this is a permanent feature.