Risers are good for two things: avoiding ‘wheel bite’ and shock absorption.
Wheel bite (burn) is when the wheels rub the bottom of the deck. This happens a lot with hard landings or turns. Risers provide a little more distance between the board and the wheel preventing bite.
Risers come in a variety of thicknesses and hardnesses. Softer and thicker risers absorb more shock than thin, hard risers. Downhillers and cruisers often select thick risers to make sure their larger wheels have adequate distance from the deck. This gives them more freedom for turning and a much smoother ride. Thinner risers are often chosen by those who skate street, ramp, and park. While absorbing shock, risers also help to keep hardware from vibrating loose on rougher surfaces.
Risers aren’t required, but strongly suggested.