For Wikipedia, if you aren't logged in (with an editor's account etc.) you are hitting a cache most of the time.

And it isn't so much for the bandwidth saving, more the saving in CPU time in rendering the page up from the Wikitext source, which can be substantial. Some very big pages used to take 2-3 minutes to render because Wikitext is a rather slow ugly programming language pretending to be a document format. (A lot has been sped up after a lot of the rendering guts got moved to Lua.)