The main stiffening agent is the yellow painted portion, providing reinforcement against torsional forces.
Can only guess that all those holes were to attach the walls to. In frame 3, the hat channel forward of the dangling light has holes spaced approximately the same distances.
Seems overkill to me, but I make no claims to being a mechanical engineer! Agreed, tho: excess holes are messy.
Particularly those stippled with GSR...