In the book "The Giver," the people in the community have adopted something called "the sameness." It's an odd thing. In some ways the sameness is a good thing. By making everybody more alike, the sameness has gotten rid of things like racial bigotry. Bad weather has also been eliminated. So far so good. But the sameness has also culled the population's emotions. They no longer feel sadness or pain, but neither do they feel real joy and pleasure. There's other weird things that the sameness created. People don't see in color anymore. The Giver explained it like this to Jonas:
"Our people made that choice, the choice to go to Sameness. Before my time, before the previous time, back and back and back. We relinquished color when we relinquished sunshine and did away with difference. We gained control of many things. But we had to let go of others."
Part of the sameness resulted in a semi-loss of memory of distant past events. Wars and the resulting destruction and hardship were simply forgotten. As were good things too. The Receiver of Memory is the keeper of all that lost knowledge. He receives all of those memories and feelings from the Giver.
The Giver/Receiver is important to the community because whenever a situation arises in which the people have no previous experience with (because they don't remember), they call on the Receiver of Memory to share his knowledge of similar events and their outcomes. He provides advice on how to proceed.
A reader might think that it would be beneficial if everybody had that knowledge. Then there wouldn't be a need for the Receiver of Memory. Unfortunately, the general population of people do not want to be burdened with all of those past memories and emotions.
The Receiver of Memories in The Giver is important because his/her job is to receive all memories from the Giver, specifically memories that would go against the community idea of sameness. For the community, the Receiver of Memories holds all of the memories that would break sameness and allow for pain, sadness, and all emotions or feelings. There must be a person that keeps these memories alive for the community so they can learn from each of the memories. What they learn from the memories help them maintain their sameness, which is set to protect the community. The receiver of memories gains these memories from the Giver.