When you restart openHAB all your Items get initialized to UNDEFINED. Then as your bindings come online and events happen those Items get populated with a “real” state. If you have rules that trigger on updates, commands, or changes, those rules will be triggered as openHAB restarts (or upon editing and saving an .items file).
If you use persistence with restoreOnStartup, your Items will instead be initialized to what ever their most recent value was when openHAB was last running. This initialization does not result in any events so Rules don’t get triggered. In short, your Items will come up with a sane and real value instead of UNDEFINED.
See the following for my recommended Persistence configuration.