I assume just strengthening, but, as you say, its nothing really.
I think (cant really remember) that i ordered one too few, so left one out. Ive carried up to the legal limit of weight in mine a few times, and there's never been any bother.
As far as rust jobs on the disco go, i found the boot floor the easiest, but it seems like people sweat about it some. A new floor from paddocks etc. and a spot weld cutter from screwfix.. then a tin or two of underseal when its done. Much easier than body mounts and stuff like that.