Why are vaccines made of the own disease agent or of fragments of it?

The goal of vaccines is to artificially persuade a specific primary immune response (and the consequent formation of antibodies and memory cells) concerning a given infection or disease in order to vaccinate the individual against infections by the pathogenic agent in the future.

As each antibody does not act against a variety of antigens but instead it acts only against its exact antigen, it is essential for the immune system to make contact in some way with the antigen against which the immunization is wanted.