Well, it serves a few purposes. It's the emergency exit for the Locker area - if the fire doors close it's the only way to get out. You can see that door from the alcoves near the lockers - often mistaken for a normal exit due to required signage. It also is an emergency exit for the single riders and regular queue as well, those are less obvious double doors you'll see down in the dungeon area once you split off from the Express/Accommodation line.
And it has a fairly large storage space. Note the lower square of "Hogwarts" that's below the big tower. Those arches that make a big square:
That entire space is hollow. Up at the level of the actual arches there's a mesh/screen floor that has some lighting equipment atop it, but otherwise it's a clear open space from the ground to the sky. You can see the backside of the rockwork and the arches from in there. They store the queue extensions and such in there.