centralized configuration, monitoring and management
More AAA options
lot's more but off the top of my head that ought to be pretty good start
The 4th in that list is the biggie if you have multiple APs to setup install and manage. You can also remotely manage. Fewer trips to site.
Your time, your network security, easier VLAN configuration, guest administration.
If you cannot see what clients are on the network and their usage patterns it makes it very difficult to troubleshoot issues of bandwidth/coverage. A ZD gives you that.
Hard to imagine trying to control my APs without a browser interface...have done previously (with 20 Netgear APs - replace with 12 Ruckus 7363s...still overkill, I digress) and wouldn't ever want to go back to the "old days".
Also on the theme could add in centralised updates.
Update ZD and it pushes out the relevant firmware to the APs. So that's uploading, rebooting, checking they are online and working done from one relatively friendly interface. Again your time...how valuable is it.
Oh, I know it's way easier for me. We have numerous other sites that are zonedirector managed. I just wanted to give them a breakdown on what's missing by having standalone APs.
No band select, no smart roaming, etc...