Why does unused cargo capacity of GVWR matter at all?

GVWR of the trailer has no bearing on the tow vehicle selection. Its an overused max figure people are told to use when shopping for a truck, which leaves them a lot of cushion. But its not by any means, required.

What's nice about the higher GVWR and more CCC is that you have that option, should you ever need it. You'll have stronger trailer all around, designed to carry that higher number.

Needed? Nope. Nice? Yeah. Especially if you like both trailers and price is not a huge factor between them.