It's well-established that vaccinated people are less likely to get the virus than unvaccinated people, and experience much milder side-effects in the event they do. Thus the reason every country on earth is racing to get as many people vaccinated as possible.
Having a population that's mostly vaccinated doesn't prevent a person from getting the virus, I don't think anyone would debate that fact, but the risk of hospitals being overwhelmed with covid patients resulting from a super-spreader are much less in an environment where the majority of people are vaccinated, for the mere fact that the symptoms are so much milder.
When the hospitals are overwhelmed (over capacity) - then everyone's at risk. This has already happened once in Thailand caused by the rapid spread of the highly contagious Delta Variant and I don't think anyone wants to see a rerun.