Disabled can mean that a zone isn’t set up. It’s also useful when you have a zone that is set up and you don’t want it running for some reason. Rather than turning schedules off and on, you can just disable a zone, and then enable it when you want it running again.
When you view all the zones, you can either have the zone view show or hide disabled zones (for example, I have a 16 zone system and four of my zones are unused and not setup, so I leave them hidden.