Immunization shots, or vaccinations, are essential because they protect against things like measles, mumps, rubella, hepatitis B, polio, diphtheria, tetanus and pertussis (whooping cough). Immunizations are important for adults as well as for children.
Your immune system helps your body fight germs by producing substances to combat them. Once it does, the immune system "remembers" the germ and can fight it again. Vaccines contain germs that have been killed or weakened. When given to a healthy person, the vaccine triggers the immune system to respond and build immunity.
Before vaccines, people became immune by actually getting a disease and surviving it. Immunizations are an easier and less risky way to become immune.