1. just some cases: this option is useful if you have background images defined in your CSS as resources on you server's filesystem and if those are rather small. In that case autoptimize will read the image and put it inline in the CSS (base64-encoded)
2. if you have a CDN (content delivery network), this option will allow Autoptimize refer to the autoptimized files on the CDN instead of on your "origin" server.
hope this helps,
frank