It has nothing to do with spreading the message.
It is all about keeping witnesses busy and "counting time".
(On the point of keeping witnesses busy, it is just like the "game" we play with kids to try and keep them happy and quiet on a car trip. "Lets see who can stay quiet the longest, and they will be the winner of a prize!". Some kids fall for that...)
Oh, also it creates a status amongst witnesses as to who "qualifies" to do the cart work.