Without the wheels, while rolling over a uneven surface, as the front of the tractor goes up over a rise, the rear mounted rake or blade will go down and dig into a already low spot. With the guide wheels on behind the rake, the rake will only dig into the high spots and drag the debris to the low spots. Just like a road grader does with the blade located between the front and rear wheels.
On the rake I built for my garden tractor I don't have wheels. I really only use this rake for collecting branches and debris under and around the trees. It does dig into the dirt as I drive up over uneven ground. The rake I am building for the big tractor will have wheels.