Why should you use multi factor authentication to secure your computer accounts?

A username and password is commonly used to access computer systems or services (eg email). However, should an attacker gain knowledge of your username and password they can very easily impersonate you and compromise your account. 

Multi Factor authentication helps protect your account by requiring you to enter a username and password but also provide another factor (such as something you have) to confirm your identity. This might be a physical security token such as a Yubikey or perhaps a biometric identifier such as a fingerprint. You can only access your account when you have both something you know (your password) and something you have (your token).