As in you are the only one in a room full of people who has food or a snack or a dessert, and everyone else has to just watch you eat, even though they may be hungry too?

I think that it came from a time when food was not nearly as abundant as it is today. Some people couldn’t afford to eat 3 times a day, or could only afford to eat very little. In other words, more people were hungry back then than they are today. In that case it was decidedly rude to eat in front of others.
Is that still true today? I think it is, and not just because others could be hungry. For example, a person brings a yummy cupcake to work and eats it during a meeting. Would you consider that rude?