why does the community need a receiver?

The receiver is actually the Giver. His job is Receiver of Memory. The Giver holds the collective memories of the community. People don't have to feel bad experiences or experiences from the past. As a result they also can't handle any serious issues or intense emotional experiences. The giver shares the wisdom of all his memories and pain with the community. The community feels he is necessary to avoid pain but they are really just enabling adults to have the emotional maturity of a child.