Slobber straps can take a lot of everyday wear and tear off of expensive reins.

For example, if a horse takes a drink while wearing reins with slobber straps, the slobber straps will help keep the reins out of the water. They are designed to be easily removed and replaced.
Some reins can be very lightweight. Lightweight reins, when picked up on and released by a rider, may be so light that both horse and rider don't get a good "feel" through them.

Slobber straps can add enough weight that both horse and rider can more easily feel communication through the reins.