Passwords. We all have little corners of our lives password protected. In many cases our on-screen prompts give us the 'remember me' option. Now one of the best ways to remember something is repetition. That's how I know my debit card number is no hang on you don't catch me out that easily.
So if someone teaches me a new skill and I don't use it for six months I have to be retaught. The CPAS telephone switchboard lunchtime rota comes to mind.
So if it is ever possible that your computer will somehow forget your password it has promised to remember, through crash, system change or any one of the other thousand natural shocks that IT is heir to, then you will find you don't know it and will have to convince some distant call-centre that you do know your mother's bowling average or the address of your first dog before you will be let back in. You don't want to have to do that. Type your passwords out manually every day; or perhaps womanually.