Upstream: different header lists for cached and uncached requests. The upstream modules remove and alter a number of client headers before sending the request to upstream. This set of headers is smaller or even empty when cache is disabled. It's still possible that a request in a cache-enabled location is uncached, for example, if cache entry counter is below min_uses. In this case it's better to alter a smaller set of headers and pass more client headers to backend unchanged. One of the benefits is enabling server-side byte ranges in such requests.