Rein bows help keep a consistent length of rein so both reins remain the same length especially for a more secure grip.
They will help your position and give you more strength with less effort. When the riders hands are in the correct position, this forces the rider to develop a correct leg position instead of balancing off the horses neck or mouth.
When using Rein-Bows during a lesson, concentrating on your rein length is taken out of the equation and allows the instructor to work on other aspects of your riding. With extended use of Rein-Bows, the rider will develop a natural feel and memory of where the hands belong while unfolding an appreciation for the effect it has on riders contact with the horse.
In wet weather they are designed not to slip on rubber reins under any condition.