Usually people coming into a country have to be vaccinated to the threats of that country, but a mobile population is one reason there is such a strong cross border or even worldwide push for vaccination (which is one of the reasons Polio vaccines were pushed so hard in places like India, having huge populations like that immunized protects people in the US)
One of the main concerns is that NOT EVERYONE can be vaccinated. A very small segment of the population is often not able to be for various medical reasons, but seperate from them are At Risk populations like children and the elderly.
Many Vaccines are not able to be administered until a child has reached a certain age, and until then, contact with adults who are potential carriers is a real concern. For some vaccines which are administered in parts, even after the first innoculation the child is still vulnerable.
For the other half of the group, the Elderly, everything is a threat to a weakened immune system. The same goes for people who are already ill.
So, why would I, a Vaccinated person, be concerned about your not being vaccinated? Because we both might have children and parents. This is the central missing piece in most of these discussions, people don't exist in a vacuum, and the ESPECIALLY don't exist seperately when it comes to disease.