Depends on the PSU...like the one u showed its really one rail split into 2 or however many, 3,4,8 w/e
Really its to stop overvoltage protection. If one rail is overloaded too much the psu shuts down for protection. Generally this is done by setting a limit on the rail, 18A, 24A w/e the specs are. But as some PSUs show they dont even have this limitation like in a review of the Corsair 520HX. He raised one rail above the supposed "18A" and nothing happended.
In a TRUE dual rail PSU (Corsair HX1000), u basically just have 2 separate PSUs powering 2 separate rails. Its all basically for protection/gimmick
Here u can read this...Single vs. Multiple from the Pro