Site Bundles are a convenient way to create backups and migrate your site(s) from on instance of Mura to another. Mura gathers all of your site's assets, including uploaded files, themes, and content to create a "Site Bundle" exported as a .ZIP file. The file can then be stored on an offsite hard drive as a backup, or handed off to a client as a completed project that can then be imported into another instance of Mura.
As you can see, site bundles can also be a great way to distribute your theme. Imagine how awesome it would be to have your theme files done, as well as a fully functional website, with sample content, and everything else so that anyone who uses your theme, will be able to see it exactly the way you intended it to be.