If Internet connection is down, but Safari has the requested HTML in its cache, instead of showing "Network diagnostics" button it could also show "Show from cache". This would allow returning to a page one recently viewed.
I'm visiting India, and we just had an electric blackout. I would have wanted to carry on reading the economist page I had open earlier. History still has it, but there's no way in Safari to read the cache copy instead of the online.
The same issue also happens if you are travelling, and don't have wlan connection. Ability to browse the sites kept in cache would sometimes make the difference in finding crucial information or not.