Why do people say R.I.P?

This Rest In Peace business never made much sense to me. Presumably, those who say such things feel that their words/thoughts will increase the likelihood that the dead person's soul goes to heaven or at least stays out of hell. I remember reading the TOPIX board when Michael Jackson passed on in 2009 and several people made remarks to the effect that now was the time for all Jackson fans to do something and pray for his soul.

My thoughts were that no human being knew then or knows now what kind of person Michael Jackson was in life. Only God knows the full story and His decision is based on perfect knowledge. At least that's what most religions teach.

It seems unfair to me that God would base a divine decision on a soul based on the size and dedication of his earthly fan club. It may be that Michael Jackson did the horrible things he was accused of although I believe he was innocent. It may be that Jackson did far worse things than anybody ever imagined. Only God knows for certain and He won't select souls based on their popularity on Earth or the fervency of their supporters' hopes.