Here’s an example of how to chain layouts. As @rdyar mentioned, it all comes down to preference and how much you want to repeat yourself.
If you chain the layouts you can have a default.html layout that has header, footer, etc. includes. Then you can build child layouts that reference that, where you may have subtle differences in the main part of the layout.
This is nice because then you’re not doing
{% include header.html %}
{% include footer.html %}
In each of your layouts. Those includes happen in default.html which cascades down to any child layouts the reference it. It’s less code, less repeating yourself, and less chance for errors.