The "backoffice-disable" setting:

If backoffice-disable is true, then the automatic backoffice processing is disabled. Automatic backoffice processing is the backoffice work that normally runs after each web page is rendered. Backoffice processing that is triggered by the "fossil backoffice" command is unaffected by this setting.

Backoffice processing does things such as delivering email notifications. So if this setting is true, and if there is no cron job periodically running "fossil backoffice", email notifications and other work normally done by the backoffice will not occur.