It's win-win. If a webmaster knows that traffic is coming in for a specific term and how long that traffic sticks around, they can decide whether they have sufficient content there to keep those users happy. If not, they can add content or tweak the site, which benefits the user.
If a user comes across a site because of a google search, it's more likely than not that he's there for a specific purpose and does not plan on returning. If the webmaster is able to glean useful information from his behavior and use that to improve his site, then it's possible that users in general are better off, but the original user is long gone.
