Grind wheels, as the name implies, are made for grinding in aggressive skating, they are not for rolling on. This is also why the wheels are significantly harder than normal aggressive wheels.
The purpose of the grind wheels is to stabilize your grind making it easier for you to keep balance. The grind wheels also provide some protection for your aggressive frames from wear and tear.
The diameter of grind wheels is smaller than normal wheels and they are placed as the two center wheels on the frame (on an anti-rocker setup).
Notice: Since the grind wheels are not for rolling on, some are being made to use without bearings, this is stated in the product text.