Vaccines are not perfect.
Kid A is infested with thing, maybe with mutations of the thing.
Kid A goes to public park with Kids B-Z and their parents.
Are kids B-Z 100% protected from Kid A’s infestation?
No. There’s gaps in every shield.
Herd immunity (also called community immunity) is an important mechanism by which the larger community is protected. For some diseases, if enough people are immune then transmission of the disease is reduced or eliminated. This is particularly so for diseases such as rubella and pneumococcal disease. High vaccine coverage must be maintained in order to prevent the disease re-entering the population.