Why set up boot priority?
When your computer boots its operating system at default it
boots it from the Hard Drive.
If you wanted to run a different operating system from either
the USB port or CD spot via dual booting you would need to boot
that before your main OS starts from your Hard Drive.
To fix this, in your bios you can select where you want the
computer to look for an operating system first, so it starts the
correct one.