Tracking all the available data on the arrival of your visitors, and tracking their on site activities, provides you with information about the users themselves, but also tells you which pages are performing well, and which ones poorly.
Visitors arrive with the search criteria they used to find you wrapped up in their referring URL. You don't need to guess what search terms are working. You can know with certainty. And, tracking them throughout their stay on your domain can tell you all the pages they've gone to, in what order, for how long, and more.
User Data
By storing all the available information in a database, you can get customized reports that show details by user, by page, by path, by all users who arrived on a certain page... You get the idea. The reporting options are configurable, limited only by the skill and imagination of your applications programmer.
Here we open the topic of "Testing", which all serious marketers, real world and online, will recommend to you as an essential and necessary process. The most popular testing type is the "Split Test". A simple example is when visitors brought to you by a single keyword are routed to two or more pages with different content, options and/or features. Tracking tells you which of the pages is more successful.
What constitutes "success" may vary, however. For arrivals in the interest phase of the buying cycle, perhaps your goal will be to get them signed up on your mailing list. That's easy enough to track. Less defined goals may be a bit more difficult.