Why do cyclists begin a race with some in front, some in back?

Question

What's the advantage to starting (and continuing) a cycling race in the front of the group? What about starting in the back? How do cyclists determine what order to start?

I'm wondering because cycling races often begin/continue in a clump (as shown here: http://www.poleposition.ag/Kopie von Cycling_BikeRace02.jpg).

Answer 1

as davd c saaid and it also help them pace themselves and to help show them what they have to do to do well in the race

Answer 2

the reason they do that is they are drafting the other there for the rider in the back saves energy so when the guy up front gets tired then they can switch