Some folks hear that Walt has built a family fallout shelter, so they go to ask him about it. He then spends the rest of this film building it and talking to the camera.
I grew up on the South Shore of Long Island, where digging down would get you water. I don't think anyone in our area had a family fallout shelter. Nor did we wonder what we would do in the aftermath of a nuclear war. Die, probably. Perhaps that's why alternate uses are mentioned; some one mentions it as a cyclone cellar, which would be fine in the parts of the country where tornadoes occur and you didn't want to go flying with your house to Oz. Another mentions using it as a guest bed room.
Those are fine, and it raises the question of the involvement of the National Concrete Masonary Association. Was this an excuse to sell more concrete masonary, as THE HOUSE IN THE MIDDLE seems to have been intended to sell house paint?
I think it was.