Research indicates that the first manufacturer to introduce a flat steel key was Briggs & Stratton. And while the first inclination might be that the flat steel key was a part of the evolution from magneto plugs to "standard" keys, it actually appears that this type of key was presented as an alternative between the higher priced standard keys and the lower cost "no-key" choice, much like the "good, better, best" approach..
They Look Like Real Keys to Me !
It is probable that the design of these types of keys was meant to convey the sense that these were "real" keys that went to "real" locks. After all, warded locks and lever locks had been using keys which looked like these for decades. What most people didn't know was that, for the most part, the cuts in these keys were meaningless, except as a means for keeping the key from falling out of the switch while driving.