The community in which Jonas and the Giver live has adopted "sameness" as a means of making life as predictable and controlled as possible. Part of achieving this condition involved the citizens giving up many individual aspects of their lives. The Giver explained 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.
This is seen as a positive and comforting thing by the citizens of the community, who don't have to make decisions or be concerned about planning for the future. The Committee of Elders handles those processes on behalf of the community.
However, there are times when the Committee is faced with an unknown situation or a decision with which includes unfamiliar circumstances. At these times, the Committee calls on the Receiver of Memories, whose knowledge of past times and events allows the Receiver to evaluate and provide advice about how to address the question.
It is important that memories of the past, of other ways of doing things and other experiences are preserved so that they can be called upon when needed. However, the community doesn't want to be burdened with the responsibility and the weight of all that knowledge, so it is the assignment of the Receiver of Memories to serve that role.
i read the book a couple of years ago but i do remember that it was important to have the receiver of memories because all the knowledge over the years have been pasted down to someone new and the guy teaching jonas has gotten very old and very weak and is slowly losing all the memories he has and they HAVE to keep all the memories even the ones that everyone doesnt even know about and also it is so that the elders can use him in times of great sorror or something like that...hope its helpful :)