Ever been to a friend's place and needed to use their computer for just a bit? You get on, then log off their Gmail, log off their Facebook etc just so you can log into the same websites.
Next time you need to do that, try this: Use the browser's incognito or private browsing mode (CTRL+SHIFT+P for Firefox or CTRL+SHIFT+N for Chrome). This way, it keeps the existing user logged in to their email or Facebook, and you can still log in. It won't keep track of your history or passwords and it is a quicker and hassle free way to use someone's computer without having to log them out.