I'm not sure that many people really do; the perspective is probably skewed because several "power users" on this forum do, and there's a lot of talk about how to connect them. 
You can find a discussion of this in several threads. There are a few reasons some people want to. If you have large Zigbee or Z-Wave networks--in terms of either number of devices or total area--a second hub may help with either reducing load on one hub (really probably not an issue for most people and if it was I'd start with reducing the problem, like excessively chatty power-reporting Z-Wave devices I see) or effectively extending the range (not really extending--it creates a separate network, so some people use this for detached garages or outbuildings that normal Z-Wave and Zigbee networks with repeaters can't reliably reach).
There is one reason why you may want a separate hub even if none of the above applies: smart bulbs. You can find dozens of threads here and in other forums discussing the poor behavior of most Zigbee smart bulbs as repeaters for non-bulb Zigbee devices (and almost as many people denying this reality and wondering why they're having problems, which then get resolved when they remove the bulbs). The only for-sure exception is Sengled, which does not repeat. The other alternative here is using a Hue Bridge to keep them on a separate Zigbee network, but that won't work with some bulbs like Osram/Sylvania that are ZHA-only in the US (Hue requires ZLL or, supposedly, Zigbee 3.0).
Another reason you may want one: custom code. Some custom (user) apps/drivers are poorly written and may cause hub slowdowns. Obviously the best thing to do would be to avoid those or for the author to fix the problems, but sometimes they are hard to track down. (WebCoRE is probably better now, but there was a time when this massive app was a frequent culprit here.) If you want to run custom code with reckless abandon, a second hub might be a safe place to do that so you don't affect your normal use of devices and "official" or at least known-good code.
Finally, some of us have second hubs for testing or development. I experiment with writing a lot of my custom apps and drivers on a dedicated hub so I don't accidentally break my main hub while doing so. (A separate hub was indispensable when writing CoCoHue.)
I am quite unusual in that I have four hubs: my "main coordinator" hub now dedicated to Z-Wave, custom code, and general coordination (setting modes, etc.); my "main lighting" hub that's really everything ZHA (most of which are motion sensors for lighting, but it also includes contact sensors, my old thermostat, and everything Zigbee so I don't have to run two networks)--I have as little custom code on this as possible and do only motion-lighting and similar automations in the interest of keeping these critical automations as fast as possible; a "ZLL" hub I experimented with instead of a Hue Bridge (I went back to the Hue Bridge so this hub doesn't do much anymore and might become my custom-code hub instead); and a "development/testing" hub as described above. Technically, I have a fifth hub sitting in a box that I also briefly used as an additional testing hub.
Most people are probably OK with just one. (Two if you want to experiment for some reason or need it for a reason like a far-away building.)
