Why does unused cargo capacity of GVWR matter at all?

My family of 6 would have 2000 minimum above base wt, to as much as 3000 lbs. i also assumed I would travel with a full water tank. as where I was going, may not have water when I got there. Ie snoqualmie pass ski area parking lot in the middle of winter!

We were about 200 lbs a person of food, clothes, bedding, dishes etc. Now add water, batteries, propane, AC, added options etc.....

I'd get the bigger CCC/gvwr trailer, even if over the trucks limit per say.

Marty