Why Jems is in the book.
In my opinion the main reason for Jem is so that Atticus/Scout can have a more obvious contrast.
“Not suppose to even touch those trees”
He is referring to the trees in Boos garden. This shows us that Jem is a good metaphor for the grater majority of The Macob people. Thus the righter can provide a way for the reader to see the different paths that both scout and Jem takes.
React!