WHY AN INDEX ?
The most compelling reason for an index is that librarians, reviewers, and readers expect to find an index when they flip to the back of a major non-fiction work. The absence or poor quality of an index is more and more mentioned in book reviews. Many librarians will not order a book without an index, thinking it will be of little use to their patrons who, after all, are in the library seeking information. Some readers base their purchase on whether a book has an index.


CAN’T MY COMPUTER MAKE AN INDEX ?
In many software programs, you can click something like Insert or Generate and find Index as an option. Word-processing and page layout programs allow you to code words and generate an index. The result of this technique is not an index, however; it is a concordance. That is, a list of the actual words that appear in a manuscript.


SO ?
Well, if you are seeking every occurrence of a word, say, calico, yes, that would appear in a concordance.

But if you were looking at types of cats, the concordance program would not know that a calico is a type of cat. It takes the powers of abstraction and classification unique to the human brain to see calico and note that page number under the Cats heading. Nor will a machine say "aha! This reader might want to know about other types of cats. Perhaps I should direct them also to tortoiseshell or Himalayan."

And what about a cross-posting to feline or Egyptian deities? These cross-postings, headings, and subheadings will be written based on the book's intended purpose and audience.

You see? A computer can search and list; a human will make connections and classifications.

One more thing, and then we'll leave the poor concordances alone. They are indiscriminate. A human will read and discard irrelevant or useless information. A concordance generator will dutifully record every instance of a word whether we want it included or not. If you made no rules in your search, it would even find each the and include them.