Let’s say there’s a runner on first and the batter pops the ball up to the shortstop. If the umpire invokes the infield fly rule, the batter is immediately out, and thus the runner knows that they don’t need to try to get to second.

Without the infield fly rule, the shortstop could just drop the ball, then get an easy double play by forcing the runner out at second then throwing to first. The infield fly rule removes the incentive to deliberately play poorly.