Risers do two things. They help relieve stress from the trucks, which helps keep the deck from cracking (also called pressure cracks, which are produced by over tightening of the hardware and general wear and tear). More importantly, risers help keep the wheels from biting into the board on a hard turn, causing the board to suddenly stop, also known as wheel bite. It's a bad thing to have happen. Most risers are about 1/8" high. If you have extra large wheels, you'll want higher risers. On the other hand, if your wheels are small (52mm), then you might not need risers at all. Most people who ride street style boards with smaller wheels do not use risers. Most people use risers for longboards or cruisers because they ride fairly loose trucks with those types of set ups. It all depends on your personal preference and trial and error is the name of the game.