Playing a record at the wrong speed is absolutely vital for the art of beat matching. As not every track are the exact same BPM (Beats per minute), you need to adjust the pitch so that you can line them up correctly.

Most modern turntables, such as Technic's SL-1200s, have a variable pitch control that allow the adjustment of -8% to +8%.

See Beat Matching 101: A dj primer.