There are some situations where you'll wish you had one.

USB devices are block devices like any other. Linux doesn't care if you install or boot from a SATA disk or a USB disk, it makes no differences in that regard. The only niggle is can be BIOS support, ...

I think that one of the most useful developments of the past couple of years has been bootable USB sticks. Not just "LiveUSB" sticks, from which you can actually run Linux, although those are ...

I have installed Debian via USB hard drive (since I do not have an optical drive). For some reason when I load Linux via rEFIt it just sits at the Linux penguin. Teasing me. This is monumentally ...