During your warm-up, you are increasing your body's demand for oxygen, so your blood vessels dilate to deliver blood more quickly to working tissues. All that work increases core temperature, and your body responds by amping up sweat production. There's a lag time in there, too, when you are producing lots of heat before you are producing a lot of sweat. Getting through that lag time is important because you want your body's cooling system working at full capacity before the starting gun goes off. Interestingly, one of the long term adaptations to endurance training is a quicker sweat response. an athlete's body knows whats coming and opens the floodgates to keep core temperature from skyrocketing.
Your energy systems also need time to transition from being at rest to competing. The mechanisms for producing and processing lactate are particularly important to activate. You always do this on a small scale, but to be ready to compete, you need to kick lactate production into high gear so u=you increase the speed at which you process it.
And last, a good warm-up prepares you mentally. try to set aside day to day worries and focus on your event. Your warm-up time is not social hour; bring your headphones and turn up the volume.
Written by Chris Carmichael is Lance Armstrong's personal coach.