Imagine, for a moment, the web was not how we know it right now.
Mr. Browser tries to get a resource web.net/resource. Mr. Server gently replies with the solicited document, with a document version identification number. "Keep it well", said Mr. Server, as Mr. Browser went away to do some other things.
Mr. Browser finally opens the envelope, and finds out a website that's probably outdated. He realized the last news were from yesterday, which is why he visited Mr. Server again for an updated version of web.net/resource. During the meeting, Mr. Browser shows Mr. Server the envelope, and says "you gave this to me yesterday, but I want a new version". Mr. Server, always willing to help, reads the envelope. It was yesterday's version, already outdated, so he looks up the newest version and gives Mr. Browser a small excerpt from it. It's a smaller envelope, incapable of holding all the information of the old document, but Mr. Server says "put this document between these two lines and all will be good".
Mr. Browser complies. He reads the document as Mr. Server told him, and it all made sense now. He didn't have to carry two big envelopes this time, just a big and a small one, which helped the world not waste unnecessary paper. This made Mr. Browser happy.
Next day, Mr. Browser took a look at a small excerpt of another document he had. He knew that exact excerpt was outdated, since he had read something different in another document, so he decided meeting Mr. Server to ask for a newer part of that excerpt.
"Hello, Mr. Browser. How may I help you today?", said good Mr. Server. "Hello, Mr. Server. It's this document here, web.net/document2. Section a/b/c is wrong. Could you kindly pass me the new version?", replied Mr. Browser. Of course, Mr. Server could do that, so he gave him a small envelope containing a/b/c, as requested by Mr. Browser.
Now, we have git or other VCS software (the most basic way to implement this), we know websites will change on certain points, so why the fuck do we keep resending the whole fucking document? Can't we save the planet by wasting exponentially less bandwidth?