For any vaccine (and the corresponding disease) there are people that cannot be vaccinated, but that can have quite severe symptoms (and/or die). If a person is not vaccinated - then this person can spread the disease among those vulnerable people. How this can be problematic - here is an example. One of the reasons Sweden has so high mortality from COVID is that during the first wave elderly care homes were hit hard, because workers spread the virus (mostly unknowingly). That you have not met such people in the last 6 months depends, I guess, on your social interactions style and on the structure of the society you live in in general.