The issue is 2-fold. First and most importantly is that there are people who either cannot get vaccinated for either medical or religious reasons and there are people with immune systems so weak that the vaccination serves little use protecting them. By having high vaccination rates, it serves as a buffer to protect these people by limiting their exposure to it.

The other side of the issue is that vaccinations do not guarantee immunity from a disease. Exposure to a disease puts anyone at risk but they are much less likely to get it if they are vaccinated but they still can get it none the less.