>Why would you weld a door
Well, for one, in my office, we have an area originally conceived as a loading dock, that was converted into offices. Windows were installed to replace the bay doors, but the actual door was just welded shut, and walled over on the inside, as it was easier to do, than to re-brick the hole in the wall on the outside.
Sometimes your floor plans just change, and its easier to turn a door into a wall, than the other way around. :)
In my situation, just a few stitches provide more security than any lock installed in the door, and its just "good enough".
There's no point to welding the entire perimeter shut, as if you came prepared to cut through a few stitches, it might be easier to just cut out the whole door.
Still, its a soft spot in your perimeter, compared to a brick wall.