Hypothetically it pays for the content. I think practically most content gets ripped off and copied and has ads slapped on it. If you are using a programming language (say Python) that has useful documentation (Clojure is a counterexample; just buy Rick's book.) you are almost always better off looking up things in the manual because Google surfaces advertising sites that add nothing but mistakes, omissions, broken English, etc. I think of how 80% of the Python answers on stack overflow have something like "print x" and won't work on Py 3.

It is hard to find content which is really worthwhile on the advertising supported web. The very nature of it is that valuable content is devalued relative to spam. Like Gresham's law, bad content drives out good.