It isn't. Targeted ads are much more valuable, to the point that fully random ads have almost no value at all. Btw. you can tell. If you regularly delete cookies you get much fewer ads. They like to say that if you don't allow/keep cookies, you get the same amount of ads, just less relevant. But that just isn't the case.

If browsers had the functionality to only preserve cookies that are needed to stay logged in across sessions, that would be quite powerful. That is all the cookies I need.