A theory, the data search engines look through is less useful for making good search. Search was basically the concept that the internet is organized into directories of content (because it was before search) and that we can look through those directories to give people the answer they would get from spending hours looking through them, in half a second instead. But with search existing, content is no longer in the form of directories, which is the data search engines need to do their job, and their existence is the entire premise of what search does. We can pretend content that isn't a directory is "directory like" and use links/backlinks, but that just means that the algorithm has something to operate on. It doesn't mean that it does it correctly.