A CASUAL fan watching the second baseman or shortstop settle under a lazy pop-up and hearing the umpire shout, "Infield fly!" could be pardoned for thinking it was a silly and extraneous comment. Obviously, it is an infield fly.
In this case, though, the words are not exactly what they seem. So for those not steeped in the rule book, an explanation is in order.
The rule in question states that an infield fly is a fair fly ball (not including a line drive or an attempted bunt) which can be caught by an infielder with ordinary effort, when first and second bases, or first, second, and third, are occupied with less than two out. When the umpire declares an infield fly, the batter is automatically out, and the regular rules on a fly ball apply to all base runners.
The reason for this rule, while perhaps not obvious to the uninitiated, should be quite clear to the regular follower of the game. Its main purpose is to prevent the defensive team from making a travesty of the game by intentionally dropping pop-ups.
Without the infield fly rule, this is exactly what would happen - and with great regularity - since the defense would have nothing to lose. Whenever there is a catchable ball in the air, the base runners have to stay close to their bags to avoid being put out. Thus the defense knows that it is going to get at least one out in either of these cases: Catch the ball and the batter is out; or let it drop, pick it up, and throw to an advanced base for a forced out.
There are all sorts of possible incentives to take the latter course. For one thing, if the infielders were quick enough, they might be able to create a double play (even a triple play would be a theoretical possibility). Another motive would be if a fast runner was on base and a slow one at the plate: Catch the pop-up and the runner stays on while the batter is retired. But on a force out, you take the fast runner off the base paths in favor of the slow one.
For all of these reasons, the defense would be strongly motivated to let pop-ups fall whenever there were runners on base with less than two out.
And it is for exactly this reason - to prevent such trickery and the fiascoes it could create - that the infield fly rule was put into effect.