Comment by bluGill

Not really. An index is also a list of ideas you should search for. Search for a synonym and control-f fails, but the index will have a "see also" for that, or worst case lets you scan for interesting words without reading the whole book. The index will also leave out all the places where a word happens to be used but are not useful to someone searching for the term.

Of course a good index is hard (read expensive) to write and so many books didn't have good indexes.