...so in the example given on the page you linked to, if you wanted to first clear the array to make sure there were no other extensions being loaded, the settings would become:
...that is useful to stop default settings from showing up in your siteaccess' array settings. I think the most common use for this I have found is AvailableSiteAccessList[], which I use to prevent other siteaccesses from showing up in the options lists in the admin interface.