Identification with Christ: Baptism reenacts Jesus' death, burial, and resurrection. Going under the water represents being buried with Christ, while coming out of the water symbolizes rising to a new life with him.
Public declaration: For many believers, baptism is a public profession of their faith and a statement of their commitment to follow Jesus.
Cleansing from sin: The washing with water symbolizes the cleansing and forgiveness of sins that comes through faith.