I got a second one so I could have all my Iris V1 devices on a separate hub from my Iris V2 and Zigbee HA1.2/Zigbee 3.0/Z-Wave devices. Having the Iris V1 devices on the same hub was causing stability problems.
It isn't just for Iris devices though. The Hub Link/Link to Hub can also be used to link to other hubs from what I understand. Since I don't have any Hue bridges, Sylvania Lightify bridges, or any other bridge/hub, I can't answer how well it works.
The way it works for me is that the second slave hub shares devices connected to it with the controller hub. All the rules are set up there. The dashboard is on the controller hub. When a tile is added, it is adding a virtual device. The Hub Link gets or sends information as appropriate to the slave hub where the physical device is paired. It is pretty seamless in practice. Things just work.