Among other things...

Is capable of setting your needle if it richens/leans while the pilot has left the starting line;
Pushes it correctly (so it doesn't nose over) and hard enough so it will be airborne in 5 feet;
Advises the pilot who has cuts and what laps the other racers are on;
Advises the pilot what lap he/she is on;
Advises the pilot as his/her number of cuts (if any) and where they occurred;
Constantly tries to help the pilot find the groove;
...and of course, when to turn for #1

Also, where landed planes are located on the runway when the pilot is on landing approach.