But John was already baptizing.
Metmom asked something similar.
St. John the Baptist's was not the Christian baptism that remits sin, it was merely a sign of penance. Ritual ablutions indeed were nothing new to the Jews even before St. John; it is not like Jesus taught us how to bathe.
By giving us an example of Himself being baptized, and God making the appearance, the Lord showed that baptism that He gives is the baptism that brings us into the sonship of God.
Note, too, that Jesus has no sin, and certainly had no lack of faith in His Father nor His own divinity, so His baptism is not for His own need, and not a sign of conversion, but for our edification, so that we know that baptism saves.
And, Jesus was baptized with John's baptism and not the RC "right" formula of, "I baptize you in the name of the Father and of the Son and of the Holy Spirit". Ooops!