Why does unused cargo capacity of GVWR matter at all?

One nice thing with a high GVWR, is that you likely have heavier axles, bearings, brakes, tires, and springs. This is a good thing, rather than barely capable running gear.

If you are going to error on your tow capability, it is better to go by the trailer GVWR, rather than dry weight, plus xxxx.

In any case, common sense goes a long way.

Jerry