A few years ago I never used the index of a book. My method was to look at the table of content, identify the possibly interesting sections, and then skim. It works pretty well. Now I know that a good index can make searching faster, especially for high volumes of information. I think of a couple of keywords and look them up in the index. If they are there then that's it, I know the exact pages with the information I need. Otherwise I backtrack to my old approach.
This is true for computers too. If you need to search something often then it pays off to preprocess the data and produce an index.
Now I just (re)realized another advantage of indexing. It is good not only for readers but also for authors because it encourages consistent terminology. All too often I write programs and refer to things (in comments and code) by different names, eventually becoming confused myself.