John chapter 17 contains one of Jesus’ prayers; often called the high priestly prayer. He knows that the time has come for him to do that which he came to earth to do. Jesus spends his night in prayer, praying for himself, his disciples, and ultimately us.
The main emphasis of John 15 is abiding in Christ. Just like a branch broken from a tree, once it is removed from its source of life it is no longer able to continue to grow. We need to be sure that we stay connected to Jesus.