There are two basic reasons for baptism among Catholics.
The first reason is that, according to Catholic doctrine, all human beings are born with the stain of original sin. Because of the sin of Adam and Eve, all people are born with sin. Baptism allows a child to be cleansed of original sin and, therefore, to be able to enter Heaven if he or she dies. As the newadvent link says:
The second reason for baptism is to enter the child into the Church. It is a ceremony in which the child is officially introduced to the Church and in which its parents and godparents promise to bring it up in the Church.