Firstly because we like to run our locomotives cab first for better visibility and unless they're going the long way (or actually are going to CIY), he's going to have to run around the train more than once. while we do have procedures for "other than head end operation" where the locomotive could push, I would not be doing so with something so valuable.
Secondly, more brakes, since the five cars are nothing but dead weight.
Thirdly, the "rider car" (nothing to do with me) is partly because we're not actually supposed to couple the older types of locomotives together. Only the R156 class can be MUed together directly. It has an onboard generator for the lights. It's also where the extra, necessary crew can ride