So many one-liners available for this question..
"Well Poho, I'm fat" is my favirote.
Payload is the single reason for a truck and the one item that can cause you to lose everything you own. A manufacturers tow/payload sheet means exactly dick when you're involved in an accident. Your only supportable defense is basing what you're carrying with what's on that silly little sticker. Granted, if you add larger tires, a cap, tow rope, pack of chewing gum, they are all payload too but that's on us.
If I'm towing my small trailer with 524# tongue weight, have a full tank of gas (226#) and am alone (340#) and am involved in a serious accident, I'm safely within my 1111# payload and my insurance company and DOT won't think twice. If I do that same thing with my decked, bed rack, sliders, winch, recovery gear, etc. I'm 600# over my GVWR on a public road and DOT has the potential to charge me with the entire accident only because I should not have been on that road at that time. My Insurance company has the right to not insure me for that accident because I "wasn't operating a vehicle within manufacturers stated safe limits"
It's arguable if the truck is "safe" and can "handle" the load but all that goes out the window in a court of law. Also when someone inevitably states "towing" it will always come back to payload. you can't tow a 10K pound trailer that will have a tongue weight at best (safely) of 10% (1,000 pounds of payload) with a full tank (226#) and a naked fat guy (340#) driving and have 1500# payload rating. Of course if anyone is aware of a way to pull a trailer without a driver or gas, I'm all about being wrong.
Sorry for the soap box. I'm sure that's a bit more than you were looking for in an answer. Which is why I like the, "I'm fat" response