Why does unused cargo capacity of GVWR matter at all?

Not to go too far off-topic here.

But could someone please help me understand this thought of 1000 lbs. or more of familys' stuff? Really?

Unless we're talking about a golf cart, a full tank of freshwater, or something else out of the norm, I just don't see how our clothes, our food, and even our cooking items can even exceed 500 lbs..