The Bible says if we cast out demons by the power of God the kingdom of God has come upon you. When the kingdom of God comes upon a person the demon has to go. This is a physical demostration of the authority of God. The light expells the darkness. The chains that bound the person are broken and they are set free from the oppression of the devil.
We do not talk a lot about demons being cast out of believers. We hardly ever hear a sermon about deliverance. Some people believe that when you are born again you have become a new creation and all the demons are gone.
In my experience this is not true. Reading the Bible or discussing the problem through counselling will not cast out demons.