It's to comfort people. I mean, let's say you are a Christian. You are getting old and fear death, or will be facing death soon in some way. Wouldn't you feel better believing when you died you were able to see everyone you loved who has died again? Rather than thinking of death as nothing... I mean, even if it's not real, it gives people something to believe in when they need it most. Religion gets people through hard times. I'm a Christian myself, but I'm not very religious, like I don't go to church or anything, and I don't have much of a view point on Religion.