Good researchers cannot limit themselves to only using online search boxes.
Q: Why bother with looking through the books stacks in the library when you have access to so much online?
A: Serendipity - "finding valuable or agreeable things not sought for"
In order to do "progressive refinement" you have to know in advance which exact words will produce the refinement you seek, and it is precisely that knowledge that we lack when we are moving around in unfamiliar subject areas.
-- Thomas Mann from The Oxford Guide to Library Research (4th ed.)
A researcher MUST browse through the primary source material to be able to see what is there.
Primary sources housed in archives and manuscript collections (or even ones that have been digitized and are online) are often: