Why does enterprise Wifi use GRE tunnels between AP and controller?

I have previously used Unifi APs - in general, these tend to be pretty simple affairs - the AP grabs a IP address, clients connect to it, and they get internet connectivity. At most, you can use different VLANs for different SSIDs.

However, I'm now using Aruba APs - and they seem to establish a GRE tunnel between the AP and a local Wifi controller.

Also, they have a separate controller, through which all traffic passes, which is different ot Unifi.

I'm looking into Ruckus and they seem to do the same - and from research, it seems many Enterprise Wifi vendors (Cisco) do the same, with GRE tunnels, as well as a dedicated controller.

My question is - what are the main advantages of this approach, over the simpler Unifi/consumer-style approach?