WD-40 allegedly has a number of benefits to offer when added to gasoline.
One is that it helps clean the fuel tank. I have no idea how dirty the inside of a car's gas tank can get because I've never seen inside one. I'm sure someone knows.
It can also eliminate (not sure how) contaminants, such as water, in the gas.
Apparently, IRMV depending on the type of gas you use and the type of car you drive. However, I don't really see the benefit of adding 8oz of WD-40 (at $21.50/gal) to 16-18 gallons of gasoline. It sounds to me like a modern version of snake oil, but that's just my opinion.