Have several tabs or browsers open to the same page. Change it in one of them. Now look at any other page, it's still the old one. Now "view source" - surprise! You get the newest one you just changed!

Why this is bad If I wanted the newest page I'd fucking reload it THEN "view source". I want the old code before I changed it to something awful and wrong. I'm staring at the right one, I just want the source for THAT page and did not say "hey, go to the server, grab me the latest source and show me that".

That is, "view source" should show the source to the current page, not something else that's on the server it got it from: if I wanted that, I know how to ask for it. What I want is the old source and there's no way to get that.