All the marketing people that I've worked with have used it to do things that you could never do before. Tracking the path people take through your website, which pages they land on, what they spend time reading and what they click through, who comes back tomorrow to buy something - it shows you what works and what doesn't on a level you can't otherwise get. Mapping that onto market segments lets you further narrow this down to just work on the things that are important to the people who buy your product, when most visitors to your site are random click-through traffic that will never buy anything, and feeds back useful information to your product design process.

The degree of focus that this permits is how startups and small companies can find and target their market quickly, where a large company could just market to everybody and see who responds. It's hard to tell how much of the tech startup boom has been driven by this, but after working for and with a few of them, third party analytics is critical because you usually don't have the time or money to find your market without it. Before we had analytics the strategy was "make a lucky guess and verify it before you run out of money". Now the default strategy is "measure who is interested and go directly to them".

I'll accept that large, successful companies don't really get much out of this sort of thing - they already know who buys their product, and have the scale and resources to target everybody. They probably do use analytics just because it's there, and not because they need to.