Jesus was and is the spotless, sinless Lamb of God. He did not have to be baptized, however, I think He did, so as to lead by perfect example for us.

 Baptism is the means of identifying oneself with Christ. Thus when we are baptized, we are identifying ourselves with Christ and just as Our Heavenly Father said "This is my Son in whom I am well pleased." He will be saying likewise for us.

The Romans were mainly interested in power. Therefore I would try to portray Jesus' miracles with the hope that they  (the Romans )would realise that no human is able to do those said miracles except he be the Son of God.