I understand them perfectly, I was making a comment in view of history of PSUs, when regulations forced max 20 A per rail, resulting in 6-rail 1200W monsters.
Multi-rail PSUs have advantage of isolating devices from each other (so you can run noisy fans/pump and delicate soundcard or some exotic interface that requires stable ripple-free power source from one PSU, but the cost is you have to carefully balance current on different rails, and if you don't have right number of devices, you'll end up wasting power - you won't be able to use full current capacity of power supply. Efficiency will also suffer, since it's unlikely you'll be able to balance the rails for that sweet 75%. Single-rail has advantage of giving you always your max current capacity regardless of number of connected devices, at the cost of small crosstalk, but they are usually much higher quality. Some PSUs use multiple bridged rails, which is possibly worst choice - you have (usually) lower quality components, you have crosstalk, and small imperfections will create current flow between regulating circuitry of each rail, that will hurt service time and efficiency.