Reason Number One: They can tell you how the visitor found you:
I think it is important for both the web developer(s) and/or the marketing team to know how the user found their site. It is important to know what search engine they used, or if they were redirected from a site or maybe even an email. By looking at this information and evaluating it the web developer can better optimize the site to make sure it is search engine friendly for all search engines. You can also do statistics and see which search engines get you the most customers and then try to improve the ones that you get the least from. The marketing team can also look at this for the same reason and to determine their content for their site. It is important for these two teams to work together.
Reason Number Two: How the site is used:
Cookies can also tell you how user uses the site, which pages they go to the most and in what order. This might be important to know, to see if any pages should be combined onto others or if a new page should be added. You may also decide to add a new page section to your site and will need to evaluate how many users visit this site.