What a good question.  The short answer is contained within the question.  For the most part, Christ did not need to be baptized, at least not for the reasons that we usually become baptized members of the Church. 

So, why do we, regular human being become baptized?  We have three reasons for being baptized: for the forgiveness of our sins, for entrance into the gates of Heaven, and to be made children of God.  There are others, but those are the main ones. Jesus did not sin, not actual sins, nor did he inherit original sin.  His home was already in heaven and He would return there after accomplishing His mission on earth.  Finally, Jesus was the Son of God by the Holy Ghost, the ONLY real son of God.  All of the rest of us are adopted.  He is the original.

The way that the question is asked seems to imply that Baptism is only a matter of need, not a matter of desire.  So, let me re-phrase the question a bit.  Why would anyone desire to be baptized if they did not need cleansing from sin, or to be made a citizen of Heaven, or to be made a child of God.  What if someone alrready had those things, why might they want to be baptized anyway?

All of those things are great, and they all are very desirable for every ordinary human being.  But, in my mind there is another benefit to Baptism that we often don’t take account of.  In fact, many people are baptized and and never take advantage of the great blessing of belonging to the community of God’s people, the Church.  We, the Church are the Body of Christ, and He is the Head.  We belong to a sacred community, and more than that; we are an integral part of Him.  We are His arms and His legs.  We speak His words in this world, and we do his work.  By being baptized, Jesus incorporated himself into this community of which He is the head.

Jesus didn’t have to be Baptized like we do, but He wanted to be Baptized.  He wanted to form this community of which we are members and to lead us as His body, continuing to do His work in th world.