What are the benefits from the variable speed grinder, as opposed to the standard grinder?

The variable speed gives you options of more control or more efficient grinding, depending on your work. Diamonds like to go fast, so the faster they go, the quicker they grind the glass, but you also lose control over the grind the quicker it goes.

With a single speed, it balances grinding efficiency and control in a mid-range speed. With a variable speed, you can grind faster for large, easy-to-control surfaces, or slow down the disks for more precise control on smaller surfaces like facets.