title: What's the purpose of Twig "include with only"?
body: <p>What's really the purpose of <code>include with only</code> in Twig:</p>
  
  <pre><code>{# only the foo variable will be accessible #}
  {% include 'child.html.twig' with {'foo': 'bar'} only %}
  </code></pre>
  
  <p>Maybe some <strong>performance</strong> benefits? Or just only for avoid <strong>overriding variables</strong> in the included template? As <a href="http://twig.sensiolabs.org/doc/tags/include.html">documentation</a>:</p>
  
  <blockquote>
    <p><em>Included templates have access to the variables of the active context. You can disable access to the context by appending the only keyword.</em></p>
  </blockquote>