When setting up a new tenancy there are lot's of fields and information that can be entered or changed.
By setting up default details in settings, whenever you add a new tenancy, these defaults are automatically inserted into the new tenancy.
You can still change the tenancy details, these are simply auto-filled by default. They are not fixed for every tenancy.
These settings just save a little time especially if you are setting up lot's of tenancies.
In addition to these, the letting unit record also has a number of defaults for new tenancies.