Short answer: To inaugurate His mission to take on the sins of the world by identifying with all sinners and their sins.
What is amazing about this?
- There is a preconceived plan for the Incarnation; Jesus, the Father and the Holy Spirit have developed a plan to deal with our sins.
- The Baptism was something planned for a very long time; Isaiah some 700 years earlier predicts that John the Baptist will be born (Mal 3;1-4) and that the Suffering Servant will bear Man’s sins (Isa 53).
- Jesus, the Son of God, being totally righteous, would agree to be baptized like a common sinner.
Longer (and very comprehensive) answer by Jimmy Akins called “Why was Jesus Baptized?”, see below: