Well done analytics can give the tracker a lot of information about if the user got there from marketting or other outreach, what they did on the site, and if they took some action that's important to the site operator (buying something, creating an account, posting, logging in, whatever), and what page was the last thing they did before they left.
If you're selling stuff, and you notice a lot of people going partway through your checkout process and leaving, it might be a sign that something in the process isn't user friendly and improvement could benefit you. Especially if the behavior has changed recently.
You might see that people come in from a marketting link where you thought they'd like to buy A, but they rarely buy A, and if they do buy something, they buy B; maybe it would make sense to use that link to go to the sales page for B instead, etc.
You could even notice that people who don't run your javascript analytics still add things to the cart, and start the checkout, but never finish, and take that as something to investigate.
This isn't a defense of Google Analytics in specific, or javascript analytics in general, this could be done serverside with just a cookie to corellate across multiple visits, or a session cookie within the same visit, or instrumenting all the links and correlating that way. Javascript could be used to remove tracking tags for copying links, but have it when clicked; great if it works, not the end of the world if it doesn't.
