title: What does 'git blame' do?
body: <p>I saw a lot of questions about methods of using <code>git blame</code>, but I don't really understand them.</p>
  
  <p>I see a <kbd>Blame</kbd> button on top of files on the GitHub interface. Upon clicking it, it shows some diff with usernames on the left bar. What does that indicate?</p>
  
  <p>Why is <code>git blame</code> actually used, apart from GitHub?</p>