The skin-tight layer underneath has a smart phone-sized pocket, so nothing can slip out easily., and an extra pocket is on the backside as well. The design of two layers adds a bit of extra warmth, protection, and prevents "that bounce" that can happen with movement.
Polyester is a more breathable fabric than cotton, keeping sweat dry and preventing moisture rash. Not to mention, polyester doesn't shrink quite like cotton, so your shorts won't get unbearably tight over time!
