Why do we do that?

[FVB_Ryan]

My mind started wandering (while still being able to concentrate...i can do that....some times :-) ) after making a call the other night. A ball was hit high in the air to left field. I'm PU, watching for fair/foul, cuz it can go either way. Left fielder comes on, and makes an easy catch. I signal fair, then out. But then I got to wondering, why do we need to signal fair/foul if its caught in flight? Does fair or foul really even make a difference at that point? Once out, the runner can tag whether fair or foul. I have a feeling that someone is going to post a VERY logical explanation, and I'm gonna feel like a fool for asking such a stupid question, but I'm up for it tonight.