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Librarians would have been power users and custodians of the engine, which would be the card catalog. I remember feeling the power unleashed when I was taught to use the card catalog. It has a significant place in the history of humankind.
The premise of relevance fuels modern search engines, and overall they deliver to varying degrees. In the old days one would note the Dewey Decimal numbers on a nearby pad, and then search the aisles for just the right book. Librarians were most useful during this part of the search, something that modern search hasn't fully grasped.