At a site like Yahoo, getting six million viewers per day, ad space can thus be sold at a premium. When Time-Warner launched Pathfinder in the fall of 1994, it registered 200,000 hits in the first week. A year and half later, it was receiving 25 million hits per week. Robots, of course, aren't programmed to pay attention to ads. One more aspect of tracking, metering, and defining the users of a particular website is database gathering.
Many websites have a registration form, a questionnaire, or just a guestbook.
These, too, can generate information of value to advertisers.