It runs it, lubes it, and cleans it. Just don't spray it inside a shortwave radio.
My grandfather loved WD40. His answer to everything was "Hell, a little WD40! It'll fix anything!" One time he had a shortwave radio that didn't work. What did he do to fix it? Spray it with WD40, of course. It worked afterward. Well, it worked after my electronics whiz dad cleaned all the WD40 out of there and fixed the actual problem, but that's beside the point.
Besides [not] fixing shortwave radios, WD40 has a wide variety of practical uses, as Bikes and Beards demonstrates in this video. According to them, it's useful as a fuel, lube, gunk remover, and scratch remover.
Okay, maybe not so much as a fuel. After draining a gas tank and filling it with WD40, the engine dies pretty quickly, despite the fact that WD40 is a flammable substance. Perhaps retuning the carburetor for a mixture of pure WD40 would help. It works for E85, after all. Besides, it runs just fine after filling the tank with good old fashioned gasoline again.
As a chain lube, however, WD40 has known and proven benefits. I've used it myself to remove the old lubricant, as well as any rust spots (dual-sport riding often involves puddles, after all). Here we see a new method for lubing a chain. Simply flip the bike upside down, like you would a bicycle, and spray it on as you spin the back wheel. The addition of fire seems to help as well, for some reason that I can't understand besides looking extremely cool.
Another great use for WD40 is removing gunk that's stuck to your bike. They use a piece of gum—well, maybe 500 or so. Before getting bored, they manage to remove a great deal of the gum from the bike simply by spraying it with WD40. This bodes well for its ability to remove other types of gunk, like residue from stickers you've removed.
Unfortunately, they can't test WD40's ability to remove scuffs and scratches from paint because their test bike doesn't have any—at least, until a not at all random van comes out of nowhere and smashes into the bike. There's no way this could possibly have been scripted or planned, of course. The bike actually doesn't look bad after getting smashed by a van, but it's hard to see any scratches the crash caused in the first place.
Clearly, WD40 is something every garage needs to have. I just used it today myself to clean up my rusty camper van's frame where I'm trying to install a trailer hitch to tow my bike. You may wish to avoid some of the ways Bikes and Beards used it, though. Don't spray it inside a broken shortwave radio, either.