I have one because a hundful of devices in my rack (switches, AP, Mini PC) are single power supply while the servers are all dual.

Makes it easy to have everything connected to both UPS separately. It came in handy a few times when I had one of the UPS freak out (or when someone decides to flip the light switch that is connected to that outlet, lol). Also was helpful for keeping things only while reorganizing/moving stuff around.

Nowadays I moved everything running in the rack to a mini PC to cut down on power so its not really in use anymore.