It is important that Jonas sees the apple change because it is his first indication that he is different and his community is not what it seems.
When Jonas is growing up, he believes what everyone in his community believes. He thinks the community is perfect, all citizens are happy, and Sameness ensures no one is ever uncomfortable. The strict rules and conformity are necessary to maintain the community’s structure and happiness. When Jonas sees the apple change, he realizes nothing is what it seems.
The apple changing is the first sign Jonas is different from others and that the community is hiding a secret. Jonas is the only one who notices anything unusual about the apple. He sees it change when he is tossing it to his friend Asher.
But suddenly Jonas had noticed, following the path of the apple through the air with his eyes, that the piece of fruit had—well, this was the part that he couldn't adequately understand—the apple had changed. Just for an instant (Chapter 3).
Jonas asks Asher about it, but he notices nothing unusual about the apple. Jonas tries to bring the apple home, but he can’t see anything different about it. All that happens is that he is publicly chastised for hoarding food. The Speaker doesn’t mention him by name, but Jonas knows the Speaker is talking about him. That’s how it works in Jonas’s world. You conform, or else.
When Jonas turns Twelve, he finds out what the change was all about. At the Ceremony of Twelve, his number is skipped when he is supposed to be called for his assignment. He and the other audience members are told by the Chief Elder that he has been given an unusual assignment and has been selected Receiver of Memory. It is a job that requires special skills, personality traits, and one unique ability.
Finally, The Receiver must have one more quality, and it is one which I can only name, but not describe. I do not understand it. You members of the community will not understand it, either. Perhaps Jonas will, because the current Receiver has told us that Jonas already has this quality. He calls it the Capacity to See Beyond (Chapter 8).
Jonas realizes he does have this unique ability. He saw the apple change. He sees the faces of people in the crowd change. He learns through his training that this means he is susceptible to the memories. Only a very small percentage of people in his community are, and Jonas is one of them.
When he begins his training, Jonas finds out he is seeing color. No one else in his community does. The apple was red, and he was seeing its redness. When he receives memories, he starts to see all the colors.
As extraordinary as it is to have a special ability no one else has, akin to a magic trick, the really important thing about Jonas and the apple is that it is the first time Jonas and the reader realize something else is going on behind the community’s perfect surface. The community is much darker than Jonas realized.
As part of his training, Jonas begins to see the community's dark side and that another way of life is possible. It is the way of life he sees in the memories, a way of life that is not devoid of feelings and ruled by politeness. Jonas begins to doubt his community and is eventually forced to leave it.