title: What is pip's `--no-cache-dir` good for?
body: <p>I've recently seen the <code>--no-cache-dir</code> being used in a Docker file. I've never seen that flag before and the help is not explaining it:</p>
  
  <pre><code> --no-cache-dir              Disable the cache.
  </code></pre>
  
  <ol>
  <li>Question: <strong>What is cached?</strong></li>
  <li>Question: <strong>What is the cache used for?</strong></li>
  <li>Question: <strong>Why would I want to disable it?</strong></li>
  </ol>