title: What is the use of If-Unmodified-Since HTTP Header?
body: <p>Do you know of any practical use of If-Unmodified-Since in the wild?
  From the <a href="http://labs.apache.org/webarch/http/draft-fielding-http/p4-conditional.html#header.if-unmodified-since" rel="noreferrer">description</a>, it appears that this header was meant to help with avoiding dirty writes. i.e. update this resource only if it hasn't been modified since the last-modified-time available with client. Unlike If-Modified-Since, it doesn't seem to help with caching. Am I missing something?</p>