I was at a page, saw something interesting, wanted to come back to it (in the same session), and set the page aside in a different tab because the thing I wanted to come back to was in no way related to what I was looking for (coming across it was entirely accidental.)
Then crashing happened. The page is not in my history because I was in a mode that doesn't track that, I can't remember the exact search terms used to run across it, so on, so forth. Perhaps it is lost to me forever. but here's something I do remember: the background color of the lost page was black, the background color of almost every page I find when looking for it is white. If I could look for pages with black background color only (something that would be in the code for the page, not the text of the page) then my odds of finding the page seem to go up a lot because I can rule out most of the results I'm currently being flooded with.
But in general it seems like it would be useful for any number of reasons to be able to search for pages in a way that included their code.