Using chains in your bench press can help you develop power through the upper portion of your bench press.
What happens in this excercise is this: Chains are draped over each end of the barbell bar, with 3+ feet of the chain resting on the ground.
As you unrack the weight and lower it, the chain begins to pile up on the floor the lower down the bar gets, making it lighter. Then, as you reverse and start pushing the weight away from your chest, more chain is lifted off the ground, making it harder to lift through the upper portion of the lift.
As you can see, using bench press chains can help you develop power through the "top end" of the bench press lift.