Why is the receiver's job so vital to the book's community in The Giver?
The Receiver of Memory's position is essential to the community because he will hold the memories which have been eliminated in the supposedly utopian society. These memories of the community's past history are important to retain in someone because in times of conflict or crisis, knowing the past may be crucial to solving problems.
The Receiver is given the Capacity to Know Beyond:
"It's the memories of the whole world," [the Giver] said with a sigh. "Before you, before me, before the previous Receiver, and generations before him." (Ch. 10)
At times these "memories of the whole world," which have been erased in everyone else so that they can live in bliss, are needed by the community Elders to make a decision in which the past history (the "Beyond") must be consulted in order to not repeat mistakes.
As Jonas receives memories, he experiences feelings unknown to him. However, he also learns that the "bliss" in which he has lived before his assignment is shallow and false. For, a person cannot truly know happiness who has also not experienced pain and sorrow. All true emotion has two sides.
The Receiver is to be the only one who will hold the memories of the community's past (their history). Those memories do not exist in the minds of anyone but the Giver and the Receiver (until the Receiver takes all the memories on). When a problem arises and the leaders need to seek advice based on history, they go to the Giver for guidance.