Why do (traditional) vending machines have push-doors?

Most traditional vending machines that I've used have these "push" flaps/doors at the bottom (where the snacks/drinks are supposed to come out). Why are these there? Without push-doors, people would easily be able to see if they left something. I also find it to be quite uncomfortable to grab my snack/drink through a push-door. Why do most (traditional) vending machines have push-doors?