The -12V rail is pretty much obsolete now and is only kept on to provide backward compatibility with older hardware. Some older types of serial port circuits required both -12V and +12V voltages, but since almost no one except industrial users use serial ports anymore you as a typical home user can pretty much disregard this rail.
Funny, I was just lamenting my computer not having a serial port. Damn the average user for not using old, obscure interfaces! My kingdom for a DE-9!