Internally a layout is stored like a page and has revisions. Meaning you can make changes to a layout and publish them whenever you want or even revert to an older state.
Discussion
Last but not least the layout acts as a fragment, direct child elements placed inside a layout are rendered to the parent of the layout. Wanna use a sticky header navigation in a layout? You can.
This is one of the most crucial features that I always wanted to have in a #CMS and one of the main reasons why I started to work on Nitropage.