• Flattening the backs of chisels and the backs of plane blades.
• Flattening water stones.

If you use older tools as much as I do then the coarsest, meanest, flat abrasive surface you can find is the thing to use. This diamond stone fits the bill well. Use water as a lubricant.
After I sharpen with water stones (and I use both water and oil stones), I keep the stones flat by rubbing them on my diamond stone (starting with the finest grit stone to the coarsest). The diamond stone won't contaminate the finer grit water stones. I don't even bother washing the slurry produced on the diamond stone away. I let the slurry dry on the stone so when I use the stones next I have a thick layer of ready to use slurry. I go from fine stone to coarse so that I don't have to clean the diamond stone when I switch stones and I don't have to worry about contaminating the smooth stone with slurry from a courser stone.
I also use this stone when I need a really coarse honing stone, and for sharpening things like kitchen knives. I don't bother having an equivalent grit water or India stone since I hardly ever need to hone something that coarse. I don't stock the finer grits of diamond stone because I think for regular sharpening more traditional systems work better.
Made of single solid crystals of monocrystalline diamonds, nickel bonded to a solid steel substrate. Flat to +/- 0.002". Wear resistant and unbreakable. (Chisel not included.)