An index requires a degree of sophistication and skill lacking in a computer-generated concordance. A concordance is an alphabetical list of words with locations in the text, a somewhat rudimentary index. The computer automates the process, parsing the text and collecting the index words and the locations in the text, then writing the generated concordance to a PDF or text file. The computer greatly reduces your time and effort, but the results are far inferior, for a number of reasons.
A computer cannot distinguish between what is important and what is not. Many concepts that are mentioned only in passing are not useful to the reader, but are included along with all other concepts. Readers are quickly overwhelmed. In addition, a concordance does not cross-reference subjects. A concordance may also leave out a subject that is discussed at length without ever being mentioned by name.
Indexers can adapt and stay true to style guidelines, while a computer-generated concordance would not. While an indexer is self-reflective, visiting and revisiting the index to improve upon it, a computer merely generates a list of words contained within the text, then the work stops there.
Computers are capable of many things, but creating a high-quality index is not one of them.
For more information about the services provided by the author of this blog, see the Stellar Searches LLC website, http://www.stellarsearches.com