The above answers are correct and there are some practical considerations also. When you come out of the swim part of the race you get on the bike. Dropping down into the aerobars with your weight on the arm rests is somewhat restful. Keep in mind you've given your upper body a hard workout in the water. The saddle forward and the angle your sitting also allows you to draw from different muscles in your legs. You still have a run to do after the bike section. If you need a snack and water, do it on the bike so you don't lose time.