Proper warmup helps prevent injuries. You need to sweat a little, but don't get fatigued by the warmup. Increases the removal of lactic acid accumulated during previous workouts. Increases the efficiency of contracting muscles. Some light stretching is good to get limbered up.
Research suggests neuromuscular coordination is enhanced by warming up.
Improves coordination of individual exercises. Increases heart rate and speeds blood circulation. Increases oxygen reserves to the muscles. Be careful not to warm up too long because you don't want to use up your energy for warming up instead of getting an intense workout. A good rule of thumb is to elevate your heart rate, sweat lightly, and have an elevated body temperature.