Not much really, since they don't render as visible, and if you are using the C+ MRR series, you can cover larger areas of grid with single mesh planes.
I use mostly the 60x20, 180x20, 70x70 and the 1x1. I use them around the edges of my MRR layouts, out to a distance of 8, 5 meter grid squares.
This keeps any scenery you place along your layout edges from dropping down to your floor height, and the free camera won't zoom and crash down to the floor in Driver. (well, not as much anyway)
If you open up my Rio Grande MRR layout in surveyor, you can open the dighole layer to see what I mean
Rico