Theories abound on the reason that Jesus submitted to baptism. After all, if He was sinless as the New Testament claims, then His baptism must have held some ulterior motive. Perhaps John and Jesus plotted or conspired together to gain attention for Jesus's ministry; perhaps Jesus came as a representative of the sinful human race; perhaps He submitted to baptism as foreshadowing His death and resurrection; or perhaps His baptism made the act of baptism work for everyone else.
Each of these theories, however, misses a few key elements. For example, we have no evidence that John or Jesus spoke prior to the time of the baptism, even though they were cousins. John lived in the wilderness and only knew what sign to look for. But most of all, John's baptism was not primarily a baptism of repentance (the turning away from sin). Instead, the submersion in water identified the person with the coming Messianic Kingdom.
Those whom John baptized had already repented and sought to be joined to the coming Messiah and His reign. John, in fact, would only accept true obedience—even from the spiritual leaders of Israel. His mission was to prepare the way for Jesus to come, not to take away sin.
In fact, no ulterior motive is required. Jesus asked John to baptize Him simply as an act of obedience to God's purposes. God had given John the promise of a coming Messiah and the way to identify Him. Jesus fulfilled that promise. His baptism was simply the right thing at the right time: the last act of His private life.
Adapted from The Life and Times of Jesus the Messiah by Alfred Edersheim (Book II, Chapter XII) and from the lecture notes of Dr. Doug Bookman, professor of New Testament Exposition at Shepherds Theological Seminary (used by permission).