1. Because being baptized was one of the conditions of salvation proclaimed by Christ's authorized messengers in the first century.
Peter said, "Repent, and be baptized every one of you" (Acts 2:38). "He (Peter) commanded them to be baptized in the name of the Lord" (Acts 10:48). "He that believeth and is baptized shall be saved" (Mark 16:16).
2. Because all believers of the gospel as preached by the apostles at the first, were baptized.
"Then they that gladly received his (Peter's) words were baptized" (Acts 2:41). "And many of the Corinthians hearing believed, and were baptized" (Acts 18:8). "And when they believed Philip... they were baptized, both men and women" (Acts 8:12). Philip baptized the eunuch (Acts 8:38). Lydia was baptized and her household (Acts 16:15). The keeper of the prison was baptized, "he and all his, straightway" (Acts 16:33). "When they twelve men at Ephesus) heard this, they were baptized in the name of the Lord Jesus" (Acts 19:5).
3. Because baptism is the divinely-appointed rite for introducing believing sinners "Into Christ".
"Baptized into Christ" (Rom. 6:3). "As many of you as have been baptized into Christ have put on Christ (Gal. 3:27). "Born of water" (John 3:5).
4. Because out of Christ, there is no salvation for any sinner (and all have sinned).
In Adam all die" (I Cor. 15:22). "At that time ye were without Christ... having no hope and without God in the world" (Eph. 2:12). Gentiles "alienated from the life of God" (Eph. 4:18). "All have sinned" (Rom. 3:23). "The wages of sin is death" (Rom. 6:23). "But now in Christ Jesus, ye who sometimes were far off are made nigh" (Eph. 2:13).
5. Because Christ and the apostles were baptized.
Jesus was baptized of John in the Jordan (Mark 1:9). Paul "arose, and was baptized"(Acts 9:18). Some of the disciples of John the Baptist followed Jesus and became the apostles (see John 1:35-37).
6. Because in baptism, we obtain the forgiveness of our sins.
"Be baptized... for the remission of your sins "(Acts 2:38 "Be baptized, and wash away thy sins" (Acts 22:16). "Baptism doth also now save us, not the putting away of the filth of the flesh, but the answer of a good conscience" (1 Peter 3:21). "Purged from his old sins" (2 Peter 1:9). "The washing of water by the word" (Eph. 5:26).
WHAT IS IT TO BE BAPTIZED
But you will say, What is Baptism? The question may be answered by considering the following facts:
1. It is an action requiring the use of water.
"See, here is water; what doth hinder me to be baptised?" (Acts 8:36). "Can any man forbid water; that these should not be baptized?" (Acts 10:47). "John was baptizing in Aenon near to Salim, because there was much water there" (John 3:23).
2. It involves being buried or immersed in water.
"We are buried with him by baptism into death... planted together in the likeness of his death" (Rm 6:3-5). "Buried with him in baptism, wherein also ye are risen with him" (Col. 2:12). "Born of (Greek, out of) water" (John 3:5).
3. In other cases where the word baptism is used, it is with the idea of complete covering over with the thing or element it is related to.
"Baptized with the Holy Spirit... it filled all the house where they were sitting" (Acts 1:5, 2:2). Israel baptized "in the cloud and in the sea" (1 Cor. 10:2). Christ's baptism of suffering: it overwhelmed him (Luke 12:50).
"WHAT GOOD WILL WATER DO ME?"
None, in the sense of your enquiry. It is a question of confoming to the ordinances and appointments of God, who has purposely made use of things which are nothing in themselves, that His name may be honoured in their employment and achievement (1 Cor. 1:27-29). He has appointed and commanded baptism. It is part of His will: and we know what Jesus has said:
"HE THAT DOETH THE WILL OF MY FATHER, THE SAME is my brother, and sister, and mother."