Mandatory immunizations have been criticized for many years now. Given that the reasoning behind the immunizations speaks to the health of children and the reduction of spreading infectious diseases, many (easily) see the benefits.
Therefore, the benefits are easy to point out. by making immunizations mandatory, the spreading of disease and infection lowers dramatically. Schools have made proof of immunizations a necessary part of the enrolling process mandatory.
Outside of that, some people may have issues with mandatory immunizations based upon cultural or religious beliefs. They do not accept modern medicine as being important in life, or their religion refuses to accept modern medicine. Therefore, by mandating immunizations, they beliefs of those whose cultural or religious ideologies are ignored.
On another hand, some people simply do not like being told what to do by the government. They simply believe that they have the right to choose what to put into their bodies.