Have you ever wondered why so many large chain stores have two sets of doors? No, it is not just to store shopping carts.
Where I live, any home that is around 100 years old (that hasn't seen any renovations) will very likely have two front doors. Putting it simply, you open one door, step into a small space, and there will be another door in front of you.
(Image Sources: Image 1 | Image 2)
The space goes by many names, including: arctic entry, mud room, breeze room, vestibule, airlock, foyer, and more. For sake of simplicity, I am going to call them "entry vestibules."
Entry vestibules create a buffer between the outside and the inside of the building, preventing drafts. This can help greatly with temperature regulation in both the winter cold and summer heat. I can't find number details on energy saving, but the fact that large chains still build them may hint of their importance for money-saving.
In homes, this space also typically serves as the mud room ( a place for shoes and jackets).
In the name of "first impressions," and open concept designs, vestibules are often the first thing to go during renovations, and I think that's a real shame.
@Blair Older houses in my country also have them, and they're called just like that: vestibules. And what's interesting is that the buildings from the communist era also have them. I grew up with them being called simply "holul de la intrare" (literally just "the entry hallway"). Indeed, it's a place where everyone leaves their jackets and shoes so that they do not bring all the dirt inside the house. In fact, my studio has an entry hallway as well, but I rarely close the door fwiw.
Newer buildings, if I recall correctly, tend to blur this demarcation, as they try to turn everything in an open space (so that it looks bigger) - as such, there's usually no door that leads to that little hallway - probably because having an AC makes it less important to have another door after you get inside the house. But I saw that it's still properly marked by walls, so you know in your mind where to leave your shoes, your jacket etc.