Why do people say rest in peace?

Answer 1

Rest In Peace is the English term taken from the R.I.P abbreviation often found on headstones. The letters R.I.P actually originate from the term, "Requiescat in pace," which is a Latin prayer that means "may he (or she) rest in peace." It is commonly found on the grave of Catholics (as far back as the 18th century) because the term is derived from the burial service of the Roman Catholic church.

"Rest In Peace" is said over the grave as a prayer request for your loved one's soul to find peace in their afterlife.