We baptise as an act of obedience to the Scriptures in response to the command of Jesus: "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" (Matthew 28:19). Baptism is thus something done for those who have become - or are becoming - disciples of Jesus.
It is an outward sign of an inward work of grace in the heart of the believer.
Baptism is also a testimony of God's grace in bringing Christians into union with Christ through repentance and faith. Baptism demonstrates and declares this union by depicting the burial and resurrection of Jesus and our burial and resurrection with Him.