A practical definition of each minister depends on each individual's personal calling, in accordance with Ephesians 4:11. Generally speaking, a minister exists to serve God by serving the Lord's people, utilizing the specific spiritual gift in which they are operating.
In Acts 6:4, the apostles delegate certain duties to deacons so that they themselves may devote their attention to "prayer and to the ministry of the Word." This does not imply that a minister is limited solely to these two activities; for the deacons themselves were also ministers, and they dedicated themselves both to the tasks delegated to them and to prayer and the preaching of the Gospel.
When Acts 6:4 mentions these two functions of the apostles—prayer and the ministry of the Word—it is identifying the foundational basis upon which the minister's purpose is to be fulfilled. According to Ephesians 4:12, that purpose is "to equip the saints for the work of ministry, for the building up of the Body of Christ." We see, therefore, that—resting upon the foundations of prayer and the Word of God—the minister equips believers so that they, in turn, may carry out the work of ministry, thereby building up the Body of Christ. The minister equips others by serving, nurturing, and protecting them. A minister is not merely an employee, but a servant; not a hired hand working for wages, but one who acts out of love.
For what purpose does a minister exist? To serve and please God by serving His people, by praying and teaching the Word, and by guiding and equipping others for the work of ministry.