Why Jonas society decide that it was necessary to have such people in the giver?

The society in "The Giver" believed that having a Receiver of Memory like Jonas was necessary in order to store the collective memories of the past and have someone bear the burden of those memories for the community. By having one individual hold all the memories, the rest of the society could live in a controlled and emotionless state without the disturbance or chaos that comes with experiencing emotions and memories firsthand.