I have also wondered about negative voltages and why they exist since a voltage is a relative mesurement. For example, I see requirements for a supply having a +12 and a -12 supply. This means the 2 "rails" have a difference in potential of 24 volts. Would you be able to use two, 12 volt batteries hooked in series? For those devices that require "+12", could you not use the combined batteris' +24 with those devices and have them "grounded" to batteris-in-series +12 point. And for those requiring "-12" , use the combined batteries "0"volts as the supply and "ground" these to the "+12"?. Then can you then build a supply powered by AC the same way? Or is there something unique about the symmetry of using the AC's positive half for the "+" and the negative half for the "-". Perhaps its easier to create 2 opposite polarity sources this way and have mirror image outputs which also reflect the same imperfections.
Anyway, I am curious why there is no discussion of the "need" for negative voltages.
Thanks