For the same reason that you need a card catalogue in a library. There is lots of great and useful information in a library, but it's physically impossible to examine all the books personally. Not even the most expert web-surfer could link to all the documents in the World Wide Web. There are millions of pages and billions of words on the Web and every minute of the day, folks are posting more. The search engines and directories help you sift through all those billions of 1's and 0's to find the specific information you need.
Search engines are one of the primary ways that Internet users find web sites.
That's why a web site with a good search engine listing may see a dramatic increase in traffic. Everyone wants that good listing. Unfortunately, many web sites appear poorly in search engine rankings or may not be listed at all because they fail to consider how search engines work. AltaVista The Web is potentially a terrific place to get information on almost any topic. Doing research without leaving your desk sounds like a great idea, but all too often you end up wasting precious time chasing down useless web.