Pocketmego, I can tell you more when I get time to rummage through my collection to find the relevant comic books.
India Ink:
Essentially it was used purely for the purpose of promoting the toy. Superman really never needed a Supermobile.
There was a toy?
What India says is more-or-less on the money, then, but for the contrivance of some sort of radiation washing over the solar system which had the effect of draining Superman's powers. I'm not sure if the radiation was Green K or red sun, but whichever.... So the Man of Steel had to build a super-mobile which was pretty much a little space shuttle which he could fly around in. It was made of a super-tough metal that I think (from memory) was ripped off from Adamantium. It also mechanically duplicated most of Superman's powers, ie. it could do what he could normally do naturally with his body, if not for the radiation affecting his super-powers.
That is the "logic of its creation," but I can flesh out the details when I find the mag. What India implies is right, though... I think the super-mobile idea is of little consequence in the grand scheme of things. It read (again, from memory) like some sort of attempt to give Superman his own version of the Batmobile... Spidey's car is another of a similar type that springs to mind, but presented more honestly, if I remember rightly.
Anyhow----the gist of it was: Superman couldn't function within this temporary power-dampening radiation, so he had to build a little space shuttle thing to fly around in.