Repboards allow you to work the top end of your Bench Press, and overload the Triceps. There is no better way to develop the horseshoe muscle in your Triceps than using Repboards on the Bench Press
Repboards allow you to reduce range of motion on the Bench Press. Lifting over a reduced range of motion, means you will be able to handle heavier weights than over the full range of motion.
Repboards will have you Benchin' more weight in no time at all, more weight means more Muscle.