The harness makes it so your lights get power direct from the battery, instead of after swimming around a much longer, more convuluted maze of wiring. Also the wiring is thicker than stock.. so you get less resistance, better current flow, etc.. someone better at OHM's law than I can perhaps offer you a better explanation but that's it. The harness also makes plugging in an aftermarket set of lights super easy. Plus it's yellow, so that's gotta be worth something.
It's only $40 now? Dang I think it used to be double that.. I think I paid ~$120 for the harness, accessory light kit, & factory switch. but that was a couple years ago. There's also a guy on Yotatech who sells these (Roger?) but they're more for Runners though I suspect he could make them for an 80, heck maybe they just work. But he's got a good web site that explains the benefits if you want to take a minute to search for it.