Why do people say R.I.P?

I was probably 30 before I figured out the answer, since I really didn't think about it much. 

I would hear it or see it, and move on.

The first time I saw it, I thought it was a monogram, but it didn't match the dead person, so I wondered if it was a murder mystery.

Then I saw it again, and didn't think much about it.

It was about the fourth time I saw it that I realized it meant something, and reasoned that it was "rest in peaches". I don't know why you would rest in peaches, though. They rot kind of easy, so I think I'd rather say "rest in peace".

Any way, it isn't explained by people who say it, so your question is quite legitimate. If I say "E.R.A" to a non-baseball fan, and say "his E.R.A. is 1.29. He's a good pitcher", I shouldn't expect the non-baseball fan to know what the heck I'm talking about.

Why not?  It's as good a thing as any to say when someone makes it out of this rancid existence.

R.I.P. A careless abbreviation of REQUIESCAT IN PACE, attesting an indolent goodwill to the dead. According to the learned Dr. Drigge, however, the letters originally meant nothing more than REDUCTIS IN PULVIS.
-Ambrose Bierce

They’re politely asking the dead person to not become a flesh eating zombie or a blood sucking vampire. Becoming a ghost is acceptable as long as they can avoid throwing furniture.