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.
So I started this comment as a reply, but I felt like people would also like to hear about this as someone who cares about mud rooms when ~~pretending I can afford to buy a house~~ window shopping.
I grew up in a rural/small town area where mud rooms are still highly valued to this day. Small houses will also occasionally have mud rooms, even if it's just a weird little hallway. In this area, the mud room usually has built in shelving and enough space to put a shoe rack plus sometimes the washer/dryer. Most commonly, it's about the same sqft of a full bathroom if the washer and dryer aren't in there; only a little bigger if they are.
I usually see them with hard wood, tile or linoleum floors to make them easy to clean, and a cheap, rough rug from Menards gets thrown on top to wipe and leave shoes.
Another, and probably more common, thing is a covered and insulated all-season porch. Usually it has screened, cheap windows and spans most of the length of the side it's on. This has less insulating power than a mud room, but with the trade off being a socializing space while semi outdoors and bug free. In the 150 yo house I grew up in, it even had a bathroom because it was a farmhouse. This house didn't have any screens, it was honestly more like a mud room than a porch; however there's also a local terminology thing about size being important in the distinction between mud rooms and porches.