For a job seeker, there are a decent number of superstars out there who do great work, know all the job openings at all the companies you care about, are in contact with decision makers and help fill big jobs. (If you want a big corporate job, that can be a great way in.) There just happen to be many more recruiters who are few or none of these things.
Not that different than the world of real estate brokers, barriers to entry are relatively low so anyone can try it, but only a proportionately small group of people are truly effective at it.
On the hiring end, a really good recruiter can definitely help a company fill specific positions, particularly in a new competency, and I've met some great people that way (but personally no hires).