You only need 4 jockeys to be involved to set up certain races, especially if the remaining horses are Muffin The Mule, Dizzy The Donkey, Willy Wonky Legs, Olly Out Of Breath, and Oh Not Not Again!
I know if I was doing the same thing day in and day out, as well as socialising with all the other jockey's a nice little bonus, every now and then, will come in handy.
Also, if fixing doesn't go on, why do the stewards sometimes call the jockeys at the gates, when they know a big plunge has just occured?