In Caylus what is the point of placing a worker at the gate
You can use it several ways. 

For example, if you want to push building the castle, but not in the first place, so you can make your opponent waste a little more resources to obtain a favor. 

Also you can use it if you aren't sure how the probost will move, to make sure you will use your pawn.

Anyway, it's another way to put some preasure on your opponent, a way to have a little advantage.