There are a few reasons why someone would use two or more linked Hubitat Elevation hubs. Most common is indeed, to segregate ZLL devices - which are known to be lousy repeaters. Another is isolating cloud connected devices from local devices. And lastly, several customers use "developer " hubs linked to their production hubs.
Basically Hub Link was born based on customers' feedback that they wanted to slowly migrate their systems from SmartThings and needed a way to link the two. We have taken that idea one step further and made it possible to link two or more Hubitat Elevation hubs based on aforementioned needs 
