When finding historical scholarship (and I assume the same is true in many other fields), the point isn't to find isolated sources; the point is to find sources that are aware of each other, that have actively discussed or debated a certain topic in such a way that our understanding has developed.

So we need to know how this conversation has been going in order to step in ourselves. The most important part of secondary research is built out of the bibliographies of works we're already looking at, scholarship we've heard presented, and so on, not just generating a list out of thin air.