Originally Posted by phreejak

The reason there are increasingly more multiple rail PSUs is because it provides for more stability since the different components that draw power from the 12v rails do so at different rates. The rails will allow for a "balancing" of the draw so as to guard (as much as it can) against a rail being overwhelmed and thus, shutting down the PSU. You'll find dual, triple, quad and now, even five 12v rail systems in most PSUs - all with varying degree of amperage like from 15 to 22 amps.

It might get confusing when you see something like "quad 12v rails at 17 amps per rail with a total of 60amps". The math doesn't quite add up but that is because the rails are rated to handle that much amperage (say 17 each) but the total amperage of the PSU is slightly less.