Start here. Then come back with a more precise question.
The Catholic Church trains priests to be exorcists. They are needed by the Church. If they can show that the devil exists then they are “proving” that their God must, by necessity, also exist. It is easier to point to wars and famine as tangible signs of Satan’s existence than it is to point to the “good” that people do as evidence for any god. It is dramatic “evidence”.
Of course no mere mortal can command a demon to leave. It must be done by a trained person for it requires power. That can only be delivered by a priest who is equipped with the ability to channel the power of the Catholic God to smite the demon. Even Darth Vader is envious of their power!
Poor Darth, he was such a quite boy growing up.