I agree that since most GP owners are not true tuners, and can pick and choose from the wheels others have already tried and tested, that the chart and the backspace measurement are pretty much useless to them.
But it was made for people like me who spend hours actually doing the grunt work to figure new things out.... I used it to figure out what 9.5 inch wide Corvette wheels would fit under the fenders of an Eldorado when using one and a quarter inch adaptors.