Why do athletes feel a need to get "pumped up?"?

In another question, a jelly asked about songs that could get him pumped up for a track meet. I’m curious about this athletic habit of “pumping up” before a competition. It’s not just track meets, but in team sports where all the guys jump up and down and shout in what appears to be an effort to get pumped up for the game. You also see it in movies amongst warriors about to charge into the pickets of the enemy.

Have you ever done this? What kind of sport? How did it affect you?

Why is this necessary? Don’t you get enough adrenaline going simply by the beginning of the competition? Is this a psychogical thing or does it have an important physical result? Is it a psych-out thing where you want your opponents to get worried about how competitive and vicious and strong you are? What’s going on here?