The infield fly rule was put into place to prevent infielders from purposely dropping pop-ups to get double plays. For example, if you have runners on second and first with no outs and the guy at the plate hits a pop up in the infield, the infielder could allow the pop-up to drop, then proceed to throw onto third and second for a DP. The infield fly rule prevents this.
As for the foul tip rule, it counts as a strike if the ball is hit out of play, or directly into the catcher's mitt, but with two strikes if a foul tip is caught by the catcher it counts as a strikeout. If dropped, it simply counts as a foul ball.
Hope that helps. :dunno: