In my case (2 hubs - basement & 2nd floor) it was to hopefully make things stable as possible by distributing processing, devices and the networks. Yes each hub has it's own unique ZW/ZB networks (you can set the channel for Zigbee if you want).
You connect them via "Hub Link" and "Link to Hub" apps.
You will still have to connect each up to your regular network - if you don't have wiring to the remote hub you can use something like Powerline Adapters to extend your wired network.
The current drawback with using hublink/link to hub is the limited devices you can expose things like switches but no thermostats etc.
I try to keep all my rules and apps related to devices connected to each hub on those specific hubs - meaning my 2nd floor hub rules/apps control my 2nd floor devices and my basement hub rules apps control my basement, first floor and garage devices. I only have certain virtual devices that talk between hubs.
In your case it may be a little trickier since you will likely have more devices communicating between hubs - I know there are people here that have done it that way..
