In a nutshell and layman:
You need the cloud key if you wish to have hybrid cloud control for your network.
The cloud key works like a man in the middle between your home network and the internet.
With the cloud key, you can access your home network from anywhere in the world as the cloud key is "hosted" by Ubiquiti cloud server (AWS). [free for up to 10 cloud key per Ubiquiti account]
For more in-depth users, some may opt to use their computers/servers as their controller and enable cloud access to do the "cloud key" job, may not be workable for some and home users.
It all depends on each's appetite.
Ubiquiti has solutions for "almost" all possible scenario, depending on the System installer skill.
Walk around outside almost any MRT stations and those LTA camera's monitoring illegal parking on the roadside are almost all using Ubiquiti wireless bridging.
If I have usg do I still need cloud key to host the controller? Thanks
-UCT