Jesus said in Mark 16:17. These signs shall follow those who believe. In my name, they shall cast out demons.
The Kingdom of God manifests through the casting out of demons. If demons stay inside, so does sin, and that is bad news.
Jesus said: "But if I drive out demons by the Spirit of God, then the kingdom of God has come upon you." (Matthew 12:28)
The Kingdom of God is the place where Jesus is King. While sin reigns in the heart and life, it isn't the Kingdom of God.
If you never cast out demons, perhaps you should question whether you really believe the whole gospel.
And if you don't believe the gospel, please don't carry on as if you have got your act together and are so much wiser than others. You may be puffed up with head knowledge, but you still need to go into the school of Christ and learn the fundamentals.