For one thing, they do it to girls too ...
Just sayin'
(Actually, now that we've learned about genital mutilation, that joke isn't a funny as it used to be ...)
John the Baptist's baptism was a Jewish rite that was practiced for the forgiveness of sins and required repentance. To fulfill all righteousness, Jesus was baptized by John.
Whenever a Gentile converted to Judaism one of 3 things that was required of him was to perform this rite. It meant that he was dead to his old pagan life and was born anew, a spiritual being.
However, this was not the case after Jesus. Now baptism symbolizes the believer's identity with Jesus Christ in his death, his burial and his resurrection. Romans 6:3-5, Col:2:12
Furthermore we see the act as a specific commandment given to us from Christ Himself. In Matthew28:18-20 He said to go and make disciples baptizing them and to do as He commanded.
In Acts 2:38-41 we see that all who heard the word were baptized, but they also had, and have to continue steadfastly as well.
Also, and this is extremely important, by obeying this command, and doing it prayerfully and publicly, we become witnesses to the resurrection, and that is a powerful and gracious gift from God. Every believer should desire it.