Why would Jesus need to be baptized by John? Well... Jesus' baptism by John was the beginning of Jesus' ministry. Mark 1:1 starts out: "The BEGINNING of the gospel of Jesus Christ, the Son of God" and then Mark starts telling us about Jesus' baptism by John.
In Luke 3:23 we're told of Jesus' baptism by John and then we read: "Now Jesus himself was about thirty years old when he BEGAN HIS MINISTRY..."
Obviously, for some reason, Jesus' ministry began with His baptism.
At this point in Jewish history, water baptism served one of 3 purposes.
1st, there was the Baptism Of Repentance. This was what John the Baptist's was preaching. But of course Jesus didn't need to repent because He hadn't sinned.
The 2nd kind of baptism was for people who desired to convert to Judaism. It was a Baptism Of Conversion. If you were a Gentile who wanted to convert to Judaism, they baptized you in water. The commentary by Jamieson, Fausset and Brown explained that: "The Jews were accustomed to say of a heathen proselyte, on his public admission into the Jewish faith BY BAPTISM, that he was a new-born child." So, baptism was used when someone wanted to convert to Judaism. But Jesus had no need to convert to Judaism. He already was one. He'd been born a Jew.
So baptism in those days could be for repentance or conversion... and Jesus did not need to be baptized for those reasons. So, for what OTHER reason would a person be baptized in water back then? Well, the only other people who experienced baptism - in the Jewish faith in that day were priests. The Law dictated that especially the High Priest was to "washed with water." And the Temple had pools set aside for just that purpose.
In Leviticus 8:6 we're told that - by the instruction of God -- "Moses brought Aaron and his sons forward and washed them with water." Then, later, during that ceremony Moses "poured some of the anointing oil on Aaron's head and anointed him to consecrate him." (Leviticus 8:12) This act INITIATED Aaron's ministry as High Priest. When Aaron and his sons were washed with water and anointed with oil, they BEGAN their priesthood and were empowered to make sacrifices and to handle holy things as God's representatives. At that point (their baptism) God put His mark of
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