Purpose of an index

An index acts as a key to the wealth of information locked up in the text.

An index brings together references to a subject or concept which are otherwise scattered throughout the text and also provides access to information from alternative terms not used by the author.

An index enables new readers to locate required information  quickly and easily, but it also has a function in enabling those who have read the text to find their way back to something they remember and want to read again.