Saint Michael the Archangel, chief of the heavenly host, has four offices, or principal duties. These are: (1) combatting Satan; (2) rescuing souls from the power of the enemy, especially on death; (3) protecting the Church Militant; and (4) escorting souls to their particular judgment.
His first office is to do battle against Satan, who is continually warring against God by trying to snatch souls away from Him. Michael was the first to challenge Lucifer, the first to do battle with him and the one to cast him out of Heaven. After Jesus, exorcists call on two main saints in their ministry: Our Lady and St. Michael, as their assistance is most powerful.
His second office is to rescue souls from the grip of Satan, particularly at the hour of death. Death is a time of great suffering, as it is when the soul is most vulnerable. Not only is the person undergoing the agony of having his soul separated from his body — a truly unnatural state, as humans are created as both body and soul, and indeed that is how we shall be after the Resurrection — but the soul is most vulnerable to demonic attacks.
Satan takes advantage of this time of suffering to attack the soul in order to tempt it to despair, but St. Michael comes to the soul's help, especially if it has been a faithful soul during life. If the soul has lived a sinful life, tradition says St. Michael appears to give the soul one final chance to repent before he is escorted to his judgment.
To rescue the souls of the faithful from the power of the enemy, especially at the hour of death & escort souls to Heaven.
Saint Michael's third office is to be guardian and protector of the Church Militant here on earth. He was also patron of medieval knights during the Crusades to recapture Jerusalem from the Turks, and is guardian of the Blessed Sacrament in every tabernacle in the world. He is also the incense-bearer during every Mass, bearing up our prayers to God the Father.
Finally, St. Michael's fourth office is to call men from life and escort them to their particular judgment, whatever that might be. He is in fact the Angel of Death. In fiction and legend, the Angel of Death is depicted as a dark, menacing figure with a sickle, when in fact it is St. Michael who is given power to cut down life according to God's will and bring the soul before the judgment seat of God.