Since track cyclists are unable to switch gears during the course of a race, choosing an initial gearing becomes very important. At it's basic level, a change in gearing is a trade off of acceleration versus top-end speed. A smaller gear allows quicker acceleration, or 'jump.' This can be crucially important in races where getting a gap on an opponent can make or break the event. On the other hand, a larger gear generally gives a higher top-end, which is paramount in pursuiting/time trialing, but also important in standard scratch/points races in which the pace will be high.
Ideally, for most mass start races, a balance is achieved to allow the cyclist a good jump, but a high top-end. Without a good jump they would be gapped by their opponents when they jump, and without a good high end they'll find themselves unable to get around their opponents when the pace stays high.