Currently when selecting fonts the Google fonts button is enabled (highlighted blue) by default.
When working on a site that loads all fonts locally using the font manager, this requires editors to toggle this button every single time.
Being able to specify the default state of the Google fonts button will save a click for every instance a font is chosen and will prevent Google fonts being accidentally used by non-technical editors.
I imagine that when the linked related feature request is fulfilled then it will also cover this one.
I suppose the only distinction is that in some cases it may be preferable to still have access to Google fonts while keeping the local fonts as the default option.