Drupal 6 and Varnish Cache

Question

Is Pressflow really required for Varnish? I ask because I've set up Varnish on my server with drupal 6.30 standard, and installed the Varnish module, and it's sped up my website no end. Really amazing speed results here, and I'm getting plenty of cache hits.

So my question, what's different about the Pressflow install, and why is it required for Varnish?