There is really never a "need" for a symbolic act. But humanity loves them. They are not only performed in religious ceremonies but in many other aspects of life, work and play. There are graduations upon the completion of a degree. You can attend a recital upon the completion of aptitude in music. The All-Star game recognizes the top players chosen by fans. People get married to show their commitment. I'm sure you get my point. Maybe one symbolic ritual is pointless to some people, but it is meaningful to others.
So, Peter...to be "Born Again" is often accompanied by a baptism, or some other symbolic ritual and it is meaningful to the person who undergoes it and to those who witness it. In this case, it symbolizes a belief in Jesus and the desire to follow his Word.
Just like any other symbolic gesture that humanity has created, it is not needed but it is often done to show the meaning publicly.