Why was Jesus baptised by John the Baptist?

We know the story about Jesus being baptised: >13 Then Jesus came from Galilee to the Jordan to be baptized by John. 14 But John tried to deter him, saying, “I need to be baptized by you, and do you come to me?” 15 Jesus replied, “Let it be so now; it is proper for us to do this to fulfill all righteousness.” Then John consented. (Matthew 3:13-15) Why was it necessary for Jesus to be baptized by John the Baptist in order to "fulfill all righteousness", according to any well known Protestant theologians?