Say you go to a place where you need to use a computer that's going to be open to the public. Firefox may not be an available option, so you're stuck with whatever they have available. They may not have the computer set up to delete your temporary files. They may have the USB ports locked out. There are a whole host of issues you might run into out in the world. I can see this as a good option to protect your privacy when you're using someone else's computer and may not trust the person using it after you're done.