To follow the example set by Christ.
“At that time Jesus came from Nazareth and was baptized by John in the river.” (Mark 1:9)
Because Christ commanded it.
“Jesus said, ‘Go then, “Jesus said, ‘Go then, to all peoples everywhere and make them my disciples, baptize them in the name of the Father, the Son, and the Holy Spirit, and then teach them to obey everything that I have commanded you.” (Matthew 28:19-20)
It demonstrates that I really am a Believer.
“We know that we have come to know Him, if we obey His commands.” (I John 2:3)