Why does unused cargo capacity of GVWR matter at all?

This is why I say it is a good rule of thumb to add approx. 1200lb to the dry weight of a trailer you are considering. No need to to pass up a trailer you really like because it's GVWR is more than what your truck is rated to tow, when the trailer in question has a 4K lb CCC.! The loaded trailer will end up weighing less than the trucks rated tow capacity.