Do recruiters do anything useful?

Paraphrasing a story someone told on HN: "everyone brags that they hire only the top 1% of developers. Problem is that can't be true, if 100% of the developers here get hired eventually." What's happening is that every time they decide to hire someone, they filter out 99% of what's left on the market and find someone. Then another company filters out 99% of what's left after that, and so on… until all developers have been distributed equally among the companies.

A lot of work for nothing? Why not skip filtering?

Maybe it's that if one company doesn't bother to filter, they will end up with a worse crop over time than the others. (Assuming that filtering is possible, but that's another topic.)