My theory is...
I'd guess the reason they designed and patented their powerhead, at a time when they didn't manufacture any machine that would benefit from it, was to stop other manufacturers using the idea and selling them with their central vac / canister systems. This way, Hoover could ensure they always had the upper-hand over the central-vac design - however convenient a C-V was, the manufacturers would only ever be able to make straight-suction machines, which Hoover had already made largely obsolete.
It's a good technique; think of all the good ideas first, even if you don't intend to use them. Then patent the technology to stop other manufacturers using them, so your machine is always superior!