Google Crawler Document Adds HTTP Caching Details
Briefly

"Allow us to cache, pretty please. Caching is a critical piece of the large puzzle that is the internet. Caching allows pages to load lightning fast on revisits, it saves computing resources and thus also natural resources, and saves a tremendous amount of expensive bandwidth."
"Today, the number of requests that can be returned from local caches has decreased: 10 years ago about 0.026% of the total fetches were cacheable, which is already not that impressive; today that number is 0.017%."
Read at Search Engine Roundtable
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