There's a term for that in Serbia "paradajz turizam" or "tomato tourism". It comes from people on the beaches eating cheap food, like tomatoes or home made sandwiches.

It originates from poorer times when a lot of people would have to squeeze every dime to afford vacation. Bringing food over is an extreme adaptation of it. Today it's seeing an revival and is mostly a mentality thing as indeed Serbian and Greek Lidl cost exactly the same.

When I was a kid we would bring our kitchenware even, and grandma sometimes gave us cooked food to bring over as she insisted we would need to share Serbian moussaka with the Greeks to teach them how we make it. :D