[Zope-dev] Medusa - Apache - Squid

I often read that for "security reasons" one should use Apache or Squid in front of Zope. So simple question: why? Isn`t the Medusa Webserver secure that all? And why does zope.org uses Apache? I would like to understand the reasons behind this decision. I thought it was for loadbalancing only - but now there is ZEO - so why using Apache or Squid? Buelent