Riser and shock pads do two things -- first, they help prevent wheel-bite. This is where, when turning, your wheel touches the bottom of your board and gets stopped suddenly. As you can imagine, having a wheel on your skateboard stop suddenly is is a bad bad thing to have happen.
Shock pads and riser pads also prolong the life of the skateboard deck by reducing the strain that the deck gets from the abuse put on the trucks. Skateboards without riser pads usually get small stress cracks where the trucks meet the skateboard deck. Riser pads reduce this problem.
Many skateboarders do not like to use risers, however, because they make the skateboard taller. A taller skateboard is harder to do technical street tricks on, and even the fraction of an inch that a riser pad adds can make a difference.
