What's with the two sets of doors?

Every mall and office building seems to use two annoying sets of doors to get inside. Ugh, why make me open two sets of doors just to get into your building? It's not like the distance between the two sets of doors is big either. You open the outside door, then a few feet in you have to do it all over again. I mean some of them are a bit bigger and decorated, but do they actually do anything?

Vestibule

Vestibules and Energy Saving
Well, it turns out that these extra doors might actually have a purpose beyond decoration and annoying guests. They're apparently called "vestibules". Not being a professional anything, I can't tell you if they actually work, how they work, or how well they work. Ask your local trusted professional.

Let me think aloud for a minute though. If you look at it from a common sense perspective, you'll notice that there are two sets of doors -- duh. There should also be two walls to the left and right sides between the two doors. If there weren't walls, then it wouldn't really be a vestibule, it'd just be a junction and you'd be properly inside the building already.

A person walks in through the outside set of doors and once he/she passes it, that door should start closing. If the second set of doors is far enough away, then maybe the outside set of doors closes before the inside set of doors gets opened. Kind of like an airlock then?

Like an airlock?
After watching enough movies, you should know the basic function of an airlock. Honestly, the only thing I know about airlocks is from movies in space or under water. In those movies, the airlocks usually let divers or space people get back into a ship without blowing the ship up. They open the outside door, enter the airlock, close the outside door, and the airlock gets drained and/or pressurized. A green light turns on and/or a sound beeps, and the people usually get the all clear to open the second set of doors to get properly inside. Or someone blows the airlock and every dies in the movie -- can't remember which one that was. And then there are those other movies where someone makes a whole in a wall of the ship or plane and everything gets sucked outside, or water starts pouring in. Having never seen or experienced that, I can't say if it's true or not.

Let's say that the pouring in and spilling out thing actually does happen for a second. In the summer and winter, air is usually heated or cooled inside a building -- you can figure out which is which. That inside air is usually conditioned with things that require energy and money. If stuff gets in or spills out, the building gets warmer or cooler, and you usually don't want that because you wouldn't be conditioning the air then. Once that happens, you have to recondition that air. And you do that with what? Money.

Assuming that air does flow in and out of a building, then maybe a vestibule could help slow that flow down. Who knows, I'm not a professional.