title: What is the point of sub request at all?
body: <p>Seems most web server support <strong>subrequest</strong>.</p>
  
  <p>I found here's one question about sub request:</p>
  
  <p><a href="https://stackoverflow.com/questions/3438154/subrequest-for-php-cgi">Subrequest for PHP-CGI</a></p>
  
  <p>But what is the point of sub request at all,when is that kind of thing really useful?</p>
  
  <p>Is it defined in http protocol?</p>