If there was no infield fly rule, then the runners would have a problem. If the ball is caught, then they have to go back and tag up. But if the ball is not caught, then it immediately turns into a force play and they have to run to the next base. The defensive team could drop the ball on purpose and easily tag out the runners, getting a double or even triple play. The infield fly rule gets rid of this problem by letting the runners stay at their bases without having to wait and see if the ball will be caught.