I was the original author of this gem, so it is nice to see it here on HN, I have since passed it on the the caring hands of Semaphore since they wanted to maintain and improve it :-)

The software was built for a forum where the admin buttons was only for specific users, but the rest of the frontend was the same, so we used render_async to render some content for some users, and other content for admins, and the rest of the page could be cached statically.

We later changed it to use edge side includes with Varnish. An example of this is where you add the to your HTML:

    <esi:include src="http://example.com/1.html" alt="http://bak.example.com/2.html" onerror="continue"/>

This will make Varnish fetch the URL(s), assemble the page and present it to the client, so your backend might see more requests, but the client only sees one.