Why Does Baseball have an Infield Fly Rule?

In baseball, if there are men on first and second and less than two out, an umpire can rule that a pop fly to the infield is subject to the infield fly rule.  In this case, the batter is automatically out.   The fielder does not have to catch the ball.  It is played as if the ball was caught.  In that case, the runners can advance after tagging up.

The reason for this is that a fielder doesn't attempt to get a cheap double play.  If there were no such rule, a fielder could intentionally drop a an easy fly and double up the runners at third and second, who would be at risk for being doubled off if they strayed any distance from the base.  It is to protect the integrity of the game by letting a fielder gain an advantage by intentionally dropping the ball.

In the Cardinals - Braves Playoff game on October 5, 2012, the extra right field umpire called the infield fly rule for that purpose.  However, it really wasn't a good call, as this wasn't an easy catch, and the fielder was so deep that he could not have easily doubled up any runner.  The call was made late, but before the catch was made.  So there was no way they could have overturned it.  It is possible that the shortstop heard the umpire call the runner out, and thought it was the outfielder calling him off.

That game is being played under protest, but it is unlikely to be upheld.