Practicing circuits perhaps..

As Tweek said its mainly for practise, but it can also be used in situations where you might not be able to stop on the runway, maybe something enters the runway infront of you that hitting is not an option. Its also done alot in aircraft prooving flights where they will do pleanty of go-arounds and touch and goes.

Although it's a cheaper way of practicing landings, the t&g is primarily intended as practice in aborting a landing after the wheels have touched down. Depending on the airplane, conditions, etc., the reason for that could be one of many... gear not locked, obstruction on the runway, not enough runway left to roll out, etc., etc.