Why does the church have a preaching and teaching ministry?

After He rose from the dead and before He ascended into heaven, the Lord Jesus Christ commissioned His disciples, "Go into all the world and preach the gospel to every creature. He who believes and is baptized will be saved; but he who does not believe will be condemned." (The Gospel of Mark, 16:15,16.) Christ has commanded His servants to preach the Word of God.

The Christian is a disciple in the school of Christ.

Christ also commanded, "Go therefore and make disciples of all the nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you; and lo, I am with you always, even to the end of the age." (The Gospel of Matthew 28:19,20.) The Christian is therefore a disciple of Christ, someone who learns from Christ.

Necessary for salvation.

When the Apostle Paul wrote to the church at Rome, he reminded them of God's promise of salvation, that "whoever calls on the name of the Lord shall be saved." He follows this with a series of questions:
    How then shall they call on Him in whom they have not believed?
    And how shall they believe in Him of whom they have not heard?
    And how shall they hear without a preacher?
    And how shall they preach unless they are sent?
This leads Paul to conclude: "So then, faith comes by hearing,and hearing by the Word of God." If a man wants eternal life, he must hear the Word of God preached.

Based on the Bible alone.

What is the proper content of preaching? What should the preacher say? From where must he get his message? The preacher is not at liberty to make up his own message. He is commanded by the head of the church to preach the Word of God, as it is in the Bible, and that alone.