Structural role of the rear box sections?

[firehawk]

Just wondering - what role do, if any, the rear corner box sections which are usually hidden below the rear bumper (see the photo below) play in the structural integrity of the car's body? Would it be possible to just remove them and then weld in some sheet metal to fill the open bottom? A small triangular support could be added in its place to support the rear bumper's sides. As far as I can see, the body of the car wouldn't lose any of its rigidity, despite the rear corners being absent.

Apparently, at some point these boxed sections have rusted through on my car and have been repaired with some sheet metal. They have then been covered with body filler and it seems that the moisture trapped underneath it has already started to rust the sheet metal again.

Removing the boxes would allow water/dirt to be thrown from the rear tyres to the inside of the rear bumper but I don't see that as much of a problem on a race car that's driven exclusively on asphalt-covered circuits. I haven't found any high-res photos of the SS Work's N2 AE86 which has a tubular subframe at the back of the car but from what I can see, these boxes have been removed and just a small support added for the bumper.