I'm not sure what you mean by index numbers, but an index on anything is an invaluable tool for locating something in a hurry. Todays focus in education is not so much on memorizing formulas and knowledge, but the direct application of those formulas and knowledge. And more importantly, can you find the formula or knowledge you need without spending all day doing it? That is where an index comes in. In the back of every textbook is an index, usually arranged alphabetically, where one may look up the subject they are seeking information about, and it tells where the subject is located in the book. When one does a search on the internet, they type in keywords that the search engine quickly crossmatches with an index of known matches. This can be done very quickly, depending on the speed and access of ones internet connection. Indexes have long enjoyed popularity as directors of knowledge; the move into the internet genre employs the same concept, albeit through the medium of a search engine.