Why Christ was baptized, and why John the Baptist was baptizing, I explained at length in a presentation of Malachi I did a few years ago which is found here:
http://christogenea.org/podcasts/malachi-christogenea-talkshoe-09-30-2011I never put it in writing, because the program was extemporaneous and I will probably redo it more formally, meaning by writing out the notes, at the end of my Minor Prophets series.
Christ was baptized to fulfill the Levitical law requiring the priests to wash the sacrificial lambs. Therefore John proclaimed "Behold, the Lamb of God ..." See Leviticus chapter 1.
John the Baptist was baptizing to fulfill the Levitical law which concerns the cleansing of the Levitical priests before the Passover sacrifice. See Malachi 3:3 and Exodus chapter 30 from verse 17.
It is all symbolic of the fact that Yahweh God keeps His Own law.
Anyone else John may have baptized is collateral, because the law was nevertheless fulfilled.