It's a preference.
I have a storage drive that has Ubuntu (a Linux operating system) installed on it. I can connect it to any computer, select from the boot menu during the computer start-up, and select the drive from the list.
It is like using your own computer, except the computer is the host; providing the processor, RAM, etc. Everything is as it were. When I'm done using it, I can shut-down or restart the computer and remove it, and the computer will start up the same as it were before, as if nothing had happened. I hope that makes sense.
Let's say want to use a public computer, but don't want to use Windows (Or Mac, if it happens to be), and you had a storage drive with from your laptop or a clone (just to make it more understandable) you can connect it, reboot, and select it from the boot menu, and there all your settings and files will appear as if you were on your laptop. You do your work, or whatever, when done, restart and remove the drive, the computer boots back into Windows, the same as it were before.
Someone at campus does it all the time. It's something I've done before, but different places have different policies. I've done it at a library once, even the screen came on to enter my library card to assess the Internet when I went to Firefox worked the same, on Ubuntu.