Whay is exorcism important to Jesus' kingdom?
Jesus' manifestations of spiritual power in His Public Ministry were intended to give His followers hope in His power as Savior. His Kingdom aims at restoring man to the state of original innocence. Hence, He cured the sick, raised the dead, preached the good news, cast out demons and exercised power over natural forces, all of which were signs of His messianic authority over all of creation. The power He showed over these realities would be a sign of the Kingdom of God breaking into human history. It was the spiritual power to "bind and loose" which He later delegated to His Church before His death.
Exorcism is one of the many spiritual powers that the Lord gave to His Church. The Church has power over sickness and sin through its Sacraments of Confession and Anointing of the Sick; it proclaims the power of life over death through its of Christian Burial, etc. Saints and communities have even been known to pray for deliverance from natural disasters, plagues and chaos and have been heard by God because of their piety.
Indeed spiritual protection from the devil was important enough to the Lord that He included it as the last petition in the Lord's Prayer: "Deliver us from evil." If the Lord did not think that evil had power over human beings, He would not have asked us to pray for deliverance from evil as a regular petition in the Church's principle prayer. Praying for deliverance from evil is thus at the very center of our faith because, in praying this way, we recognize that only God can truly save us from the power of the devil. We recognize, more importantly, that we cannot save ourselves.
By Fr. Thomas J Euteneuer, "Exorcism and the Church Militant" page 8.