Sandpaper backing, why granite?

[artmichalek]

I'm not sure the human eye can tell the difference between a blade lapped on a piece of pefectly flat granite and a blade lapped on a piece of nearly perfectly flat steel, but I understand the desire to get as close as you can for the principle if no other reason.

You won't see the difference at the end so much as during the process. If the base is perfectly flat everywhere you can move up through the grits much more quickly. If on the other hand, your nearly flat steel base leaves a little bit of a hollow at 300 grit it's going to take forever to work out at 600 if you're not over exactly the same spot.