There are countless myths about single and multiple rails used in computer power supply units (PSU), but here is the best explanation I’ve seen which further opened my eyes. I used to be a fan of single rail designs because they are just easier to manage on overclocked systems, but I never thought about benefits of multiple rails. And Tech Syndicate made a great interview with someone from Corsair (they also make some pretty damn good PSU’s).
There are also other vendors like German computer hardware maker Be Quiet! that doesn’t offer switching through software, but they offer mode switching through a physical switch/jumper on the PSU itself. There might be other PSU makers as well, but I’m currently only aware of these two.