Victor helps you delete all your Joplin and start over. It can sometimes be useful for example when you want to clear a sync target you do not have full access to. In that case you would invoke Victor, delete all your data, then synchronise with the service (which in turn will delete the remote data).
Of course the plugin should be used very carefully because once deleted the data cannot be recovered. In fact, after having used it, it is better to disable or even uninstall it to prevent any accidental use.
You may also want to backup your data first (File > Export All > JEX) in case you realise later on that you had some important data in there.
In Joplin, open the configuration screen, then under "Plugins" search for "victor".
Then under the Tools menu, select Delete all data...
All notes, notebooks, attachments and tags are deleted.
Note revisions however will not be deleted and those can potentially be large. To delete them too, go to Configuration > Note History and select "Disable note history". You might want to close and restart the application to ensure the note revisions are deleted too.