Using a USB stick to protect your Mac is one of the simplest ways to prevent unauthorized use of it.
Of course, normally using a password to protect it while you are away is usually enough. However, if you ever forgot that word or it becomes corrupted, you're in big trouble. Rohos Logon Key doesn't need a password to protect your Mac. Instead, it automatically locks your Mac screen when the USB Key is unplugged and won't start again until you put it back in.
As a result of installing Rohos Logon Key, any passwords that you have setup on your Mac will be disabled so if you try it out and then uninstall it, remember that your Mac will be unprotected! It's also a good idea to make a secondary key which you can only do if you upgrade. If you lose the only key you've got, then you're obviously in big trouble.
Rohos Logon Key is an unusual security solution that saves remembering passwords but relies on you never losing your USB stick which is probably more likely.