This way, when you create a new gallery, it will automatically have the default combination of layout, theme and options applied to it. Of course, you can always go back and override these choices.
The option to Save as Default is found in the Customize Website View of groups, galleries, collections and custom pages. See this link for how to access Customize Website View.
The new default settings will be saved and applied to all of the pages in the gallery, group or collection in which the default settings are applied. So for example, if a change is made to the gallery thumbnails page and the Save as Default is clicked, all of the settings applied to all of that gallery's pages (thumbnails page, photo page, Quick Shop page and Slideshow) will be saved as the default settings for all galleries.
Any gallery that has been set to use default settings will then automatically have those settings applied to all of the pages in that gallery.
And when a new gallery is created, it will automatically have those default settings applied to the new gallery.
The pages that use default settings are the pages affected by the default setting. You can see if a page uses default settings by navigating to the page in Customize Website View and seeing if Default Settings Applied is displayed.
If it is not displayed and you would like the page to use default settings, open Theme, Layout or Options, then check the box for Use Default Settings for Layout, Theme, and Options.