'These things' that Jesus told the disciples were horrid. He told them that he is parting company with them. He said the world that they have to live will persecute them.
However, what is the purpose of telling them all these things that will happen to them? Jesus' reason is clear--that they may find peace in him. In a world that is friendly to them, they will have peace. But that will be the peace like the one we will have when there is no conflict. However, when there are conflicts, then the source of peace must be something different. In the midst of their conflict with a hostile world, the disciples have to seek Jesus for peace.
The reason for peace in Jesus while living in conflict with the world is that Jesus has overcome the world. The ultimate victory doesn't belong to the world, but to Jesus the victor. When the world deprives us peace, we can always find solace in the fact that the world is already defeated by Jesus. It is the defeated enemy that is shouting at us and fighting with us. We have peace because we already know that the enemy is not going to win because it has already lost!