Not sure if there is a benefit to combining the rails, but apparently it can be done on a lot of other PSUs as well.
On the lower right edge of the PCB we found a small switch. This switch is actually to turn off the OCP on each of the four 12v rails to "combine" them into a single one. This is a very common method as most other manufacturers approach rail separation the exact same way. Normally separated 12v rails can be generated with just these separated OCPs on each rail. They have one or two 12v rails and just split them into a few more in order to show a higher number of 12v rails.