It's read-only after you create the image. And then you can use overlayfs on top of it as others mentioned -- useful for playing with live Linux distributions o

It's read-only after you create the image. And then you can use overlayfs on top of it as others mentioned -- useful for playing with live Linux distributions or embedded systems where you never want to change the root bootable image except when you are actually updating the firmware. ...Or, in the author's case, when you have hundreds of directories that have only small differences compared to each other. As other posters in the bigger thread mentioned, this is also very useful for big arcade game collections.