Cause I'm that guy who always packs too much s**t for a trip.
The biggest reason for too much stuff is my fishing kit which includes a pack raft. I like to camp at small alpine lakes in the back country and as such need everything to survive out there with relative comfort for several days at a time. After my last trip a couple weeks back I was looking at ways to free up space in the panniers and kept looking at all that empty space on the rear seat which will never be used for the purpose it was put there for (no where for feet to go anyway as the rear pegs are off the bike too). The one thing I like about the R-GS models is the two piece seat which lets you replace the rear half with an additional luggage plate, so without wanting to cut my seat in half, the next best option seemed to be to add a plate above the seat which would allow a small bag to be tied on and still let the seat be removed.
So, I made a template and worked up the design and cut a sheet of 1/8" aluminum on my mill. A couple bends and some clamps to mount it to the rear grab rails and I now have a nice luggage shelf. I picked up a Wolfman small Rolie bag and in it fits all the tools and such I like to carry which will free up all that space in a pannier so they are not so jam packed.