Confession has to do with the verbal expression of one's faith. Paul pronounced, "The word is near you, in your mouth and in your heart" (that is, the word of faith which we preach): that if you confess with your mouth the Lord Jesus and believe in your heart that God has raised Him from the dead, you will be saved. For with the heart one believes unto righteousness, and with the mouth confession is made unto salvation" (Rom. 10:8-10).
Notice that confession is something to be done with the mouth. Notice also that confession is "unto" salvation. This is very important because many have the idea that they are saved the moment they confess. This is far from the truth. Even the Ethiopian Eunuch understood this at the point of his conversion. While Phillip and the Eunuch were riding in the chariot, they passed a water hole. The Eunuch asked, "See, here is water. What hinders me from being baptized?" (Acts 8:36) Phillip responded by saying, "If you believe with all your heart, you may." With faith bubbling over, the Eunuch answered and said, "I believe that Jesus Christ is the Son of God" (v. 37).
Clearly the Eunuch did not think he was saved yet because he asked about baptism. Also, we can see from Phillip's response that the only thing separating the Eunuch from being baptized was a verbal expression of his faith. After the Eunuch confessed, Phillip "commanded the chariot to stand still. And both Philip and the eunuch went down into the water, and he baptized him" (Acts 8:38).
And so, confession is necessary because it is the culminating act before anyone can have their sins washed away.
(Confession alone doesn't save. You must be BAPTIZED)