When you need information from a manual or a help system, how do you look for it? Many of us will use the index first, if there is one. Why?
A table of contents is the writer's view of the book. It describes the structure. It tells a story.
If you need to know one specific bit of information, you may not know exactly where you are in that story; and you probably don't care. You just need to know how to change the frabbetizing configuration settings on the MumbleCo unit, and you'd prefer to get it done before you stop and go outside for a smoke.
If your company has recently switched to this product from a competitor's product, you may not know that the appropriate terminology for the setting you want to change on this fine product is "frabbetizing configuration". You may still be thinking in terms of the competing unit from Behemoth Inc., which calls the equivalent setting on its machine "whatsit settings".
If the manual for your new MumbleCo unit has a good index, it doesn't matter whether you know where you are in the table of contents story, and it doesn't matter whether you're up to speed on MumbleCo terminology. You can look up the term you know - "whatsit settings" - and the index teaches you, very gently, "See frabbetizing configuration." And lo, when you look up frabbetizing configuration, there it is, with sub-entries that point you to all the things you might want to know about it.
If the author of the manual has given the index short shrift, you're on your own. Maybe you should take that break before you try to find the information you need.
Fairly often, the index does get short shrift, like all the other things at the back of the book. Making a good index is labor-intensive; there's no good way to automate it, regardless of what the makers of authoring tools would like you to believe. It takes a person asking "What is this paragraph about? What would I have been looking for if my search led me here?" Fairly often, the index entry is a phrase that does not occur in the indexed material. But the fact that it may take 6 to 8 hours per page to create the index does not mean you should skip it.
The index is a reader's random-access information discovery tool, and it is (or should be) the writer's teaching tool. With all due apologies to Adobe, try this experiment: Look for a specific bit of information, using generic terminology, in the help for an Adobe product. For example, in Illustrator, how do you move a vertex to change a shape? You'll see what happens when you overlook the teaching aspect of the index. In Adobe-land, you have to know the terminology already to find the help you need. But who needs the most help? People who have no prior experience with the product, and don't know the terminology. In all fairness, Microsoft is just as bad; I never have been able to figure out if there is a way to change rows to columns and vice-versa in Excel when I decide I've structured my spreadsheet badly. And I have never been able to make heads or tails of the bookkeeping software I bought last year, because I don't know the terminology. In each case, the index fails in its teaching function - it doesn't include plain language that refers me to the specialized term.
The lack of a well-designed index in a manual or help system makes the whole product less usable.
When you work out the schedule for writing a manual or help system, include the time to create an index. In my experience, it will take about a sixth of the total writing time; your mileage may vary. But no matter how much time it takes, do it. Your readers are counting on you.