It's true that vaccination has enabled us to reduce most vaccine-preventable diseases to very low levels in the US. However, some of them are still quite prevalent — even epidemic — in other parts of the world. Travelers can unknowingly bring these diseases back home to the US. If we’re not protected by vaccinations, these diseases could quickly spread throughout the population and cause epidemics here. At the same time, the relatively few cases we currently have in the US could very quickly become tens or hundreds of thousands of cases without the protection we get from vaccines. Several recent outbreaks of measles, pertussis and varicella in the US have been traced to pockets of unvaccinated children in states that allow personal belief exemptions. When vaccination rates go down, disease rates go up.
