Among other things, if you become logged out of the device or it's reset you will no longer be able to login with an amazon account ( which is required ) to use the device
You can use the Kindle without an Amazon account if you're fine with loading all your books over USB. It will give you a nag pop-up telling you to log in each time you go to the main menu, but the pop-up doesn't show up while you're in a book so it's not a big deal.