Why does unused cargo capacity of GVWR matter at all?

Then how shall advisers here on this/these forums say to newbies or anyone who
asks this type of question....use the 'dry' as basis to figure their setup/purchases?

IMHO...being over weight won't have the wheels instantly fall off...but will
sooner and dependent how much over the OEM's ratings...driving style...terrain, etc

The biggie for me...is that the performance is lessened when over weight and
again, dependent on how much over, driving style, terrain, etc...