Hopefully never. The boost feature on a SWG simply turns the unit on at 100% for 24 hours. Every once in a long while that might be useful, but for ordinary situations it is not useful.
If you are shocking the pool you want the FC level to go up quickly, which a SWG won't do. So shocking needs to start with manual additions of chlorine. When shocking you can use the SWG in boost mode to supplement the chlorine you are adding manually, though most people prefer to shock with the SWG off because it interferes with the overnight FC loss test and to save on cell lifetime.