Depending on when you do it it could be for a couple of reasons. One might be because the opposing team has 5 defensive backs in the game and you want to run at 5 defensive backs. If the QB runs off they will likely bring in another linebacker or lineman and then that will negate the blocking advantage to a degree. You also could add some trick play to this by bringing the quarterback into motion and handing it off to him. Then he can have a run or pass option. We haven't used it or done it, but it is possible.
Personally, I am not a huge fan of the wildcat. I think it is fine if on occasion you throw out of it, but I don't see much sense in it when the entire defense knows you're going to run.