As in, when would you not want the Item to autoupdate from a command.
In truth, this is the idealised condition for OpenHAB where an Item is associated with a real device. You send it a command, a binding processes that into some transfer to an external device. Some time later, when the device feels like it, or the next poll happens, an update arrives into the binding and is used to update the Item. The response from, say, a dimmer to command ON might be “100%”. The Item reflects the real world.
In reality, people want an instant response on their UI; some devices don’t produce feedback at all; we use virtual Items not associated with real devices.
So OH has an ‘autoupdate’ feature.
I find it helpful to think of autoupdate as another binding, the same process is followed of command->Item processed (eventually) into update->Item. ‘Eventually’ is fast but a finite time.
It would probably all make more sense if they had chosen autoupdate=“false” as the default setup for Items, but that’s not going to change now!
Example of rule that exploits the difference between command and update