Exterior walls are many things. They hold weather out and weather in. They are a buffer between the environment and ourselves. A poignant statement as I sit here being simultaneously roasted by the sun and chilled by the wind.
When we speak of shelter, we speak of the bubble we create separating us from the discomforts in our environment. What these discomforts may be are at once common to all humans, yet also vary widely depending on local conditions and individual tolerances.
That bubble of shelter is traditionally composed of walls, a roof, and a floor.
A floor, because a flat level surface is most comfortable to walk upon, and because the existing ground may harbor its own discomforts, like dampness, chill, bugs, etc.
A roof, to shade us from the sun and shelter us from precipitation. Also because the greatest transfer of heat energy happens vertically.
A roof joined to the floor—as in the classic A-frame tent or a dome—gives a complete bubble. It is simpler in construction, and somewhat more “natural.” Anthills and animal dens seldom have “walls.” Indeed, many primitive shelters do not have walls. And I think these are mostly structures which are more special purpose. I mean that people did not spend a majority of time in them. They slept there, stored food, etc., but by and large spent most of their time outdoors.
It seems contradictory, but walls give us space. A roof is, by definition, a horizontal covering. In order to meet the floor and still have space beneath for people, it must be domed, arched, slanted, gabled, etc. And it will always interfere with usable space where it meets the floor until it hits vertical—at which point, it is no longer “roof” and becomes wall.
To gain usable space, you can raise the roof on, say arches or pillars. But then you break your bubble. To seal that bubble requires walls.