Using before:20XX and after:20XX helps get more results if there are lots of results for your search term. It's most noticeable when searching a word that should obviously have millions of results and sorting by view count, but youtube seems to algorithmically select just a few hundred results to show you. Even vids with tens of millions of views can be missing. Even when searching something like the title of a not very popular anime that doesn't have more than a few hundred vids on youtube, it'll still hide stuff from you if you don't use before: and after:. Especially stuff with low view counts.