This was my reason. I wanted to move away from the Hue Bridge and also upgrade my Z-Wave to S2.
Since I got the C7 as main hub and my C5 as ZigBee bulb hub, my smart home have been working almost flawless.
Unintended benefit of having two hubs, when one hub became completely unreachable (error 500), having the other one available to get into my account and pull down/initiate restore was very easy. Wasn't the main reason I had one tho. I voted above.
I have only voted, not posted, so far on this thread... I think my (tongue in cheek) response to the question would be... The question should be "What are the reasons for a third or fourth hub?"
I voted for automation complexity, for the reason that I chose to move all my Hue lights onto a new HE hub to enable me to do more complex automations that were becoming more and more difficult on the Hue bridge. This, along with Hub Mesh, allowed me to introduce contacts sensors and other devices into my lighting rules.
Anyway, the reason my tongue is inserted in my cheek is that I feel like the overwhelming reason for me to purchase an additional hub, beyond the immediate reason for separation of lighting from other more intensive processing, is to have a third+ hub for development or testing of new ideas, apps or drivers, without the impact on hubs that you need to deliver a reliable outcome day to day, such as lighting and, in my case, other controls and visuals through my original C-4 hub. I currently experience this an have been tempted a couple of times to expand my setup.
My original reason was to reduce overhead on the hubs, first by splitting by location even though one hub could probably cover the whole house then by device type to isolate devices/processes - Cloud&Network/Zigbee/Z-Wave.
edit: have 3 hubs in operation - (2) C-5s and a C-7... also have several still in their boxes.