The main difference between the Global Layout and a Layout is as follows:
The default is a Global Layout. To this, you can add a Header, Page Title Bar, Content, and or a Footer Layout Section, which will display across the entire website.
Besides the Global Layout, you can create Layouts (not assigned globally), and then determine with conditional logic which pages or posts will use the Layouts. You can also create as many Layouts as you need. For example, you can have a unique layout for each page on your site, or have a unique website footer for each page. The possibilities are endless.