In addition to any energy shields protecting a ground vehicle, having at least some "armor" would make sense. The Federation's shields are neither perfect, nor impenetrable. Having a physical barrier against hand weapons, radiation, flying debris, and simply the local environment, again makes sense. The ability to switch off the shields and still enjoy a degree of protection could make the vehicle more stealthy. A 24th century Marine "tank" might have a main weapon powered not by a power pack, but a on board reactor. And carry sensors more capable than a hand held tricorder.