I love indexes. I love using a good index and I love creating a good index. All this dates back to my childhood and cookbooks. The Joy of Cooking, in those days, had an index that was truly a joy. The person who created it (perhaps Mrs. Rombauer herself) knew cooks well enough to know what they would be looking for, and gave us a long, redundant, gloriously usable index.
Mastering the Art of French Cooking, on the other hand, had an index created by a moron. Although Mastering the Art was published in two volumes, Volume II had a combined, color-coded index that always threw me. Worse, there was a confusing use of French and English terms. Look up mustard and you found some entries; look up moutarde and you found others. I bet that Julia Child passed off the indexing responsibility to a flunky.
Every couple of years Mrs Rombauer published a new edition of The Joy and she revised the recipe selection, the recipes, and the index. It was a true process of continuous excellence. Mastering the Art, on the other hand, has not been revised, except to update for new equipment (such as food processors) and changes to ingredients.
It is not uncommon for doc managers to task junior writers to index the works of senior writers. It's not just that you need to understand content to do a good job indexing it; it's also that indexing is a good exercise for a writer to do as part of their content creation. It's a second level check of your content organization. It's also a way to implement reader issues you might be aware of. For example, if you're documenting MS SQL Server and you know that some readers will be more familiar with Oracle, you can index some Oracle terms such as System ID for Database Name.
There is a growing movement in the tech writing biz that indexes are unnecessary. I see the point for online help; numerous studies (including my own) have shown that readers prefer to use Search (and in fact, they often prefer to use Search in Google rather than within the doc). But if you publish PDFs that your readers will print, then you need an index. And if you're going to have an index, you should make a good one.
At a conference years ago I attended a lecture on indexes given by an academic. His theory was that indexes should train the reader to use correct (his idea of correct) terminology. The example he gave was of a home first aid guide he had once worked on. He declared proudly that he had changed the index entry for "collar bone" to "collar bone: see clavicle". I don't know what he said after that because I walked out.