They have some hardware that runs it's management interface on the device itself. Their UniFi and USG and some of their more prosumer-ish switches and stuff require you to run the UniFi Controller, which is web-based, on a computer. Our ops guys manage our 3 UniFi APs running the UniFi Controller on their individual laptops. If they need to change configuration they fire up the UniFi Controller and hit a webpage on their localhost.
The devices, once configured, run independently of the UniFi Controller. It's just needed for configuration changes or monitoring outside of snmp. (or apparently the guest portal)