1: Filtering. The downside of the internet making it easy to apply for jobs is that the candidates that do apply, don't tend to be the candidates you want. If you put a job up on Monster / Indeed etc, you're going to get a lot of candidates who aren't remotely qualified for what you're looking for. There's a lot of sifting that needs to happen. If you're a hiring manager you don't want to spend 20 hours initially qualifying and disqualifying candidates, when you can spend 2-3 hours talking to the most qualified people.
2: Talent attraction. A good recruiter finds out what someone wants in their career and then matches them with an opportunity. This sort of discovery doesn't happen that well on the internet.
3: Candidate pipeline management: A lot of companies have stupid complicated interview processes and end up losing a lot of talent that way. If you aren't Google, you can't put people through that sort of thing (and even Google lose a lot of candidates like that).
By and large HN doesn't understand sales particularly well. Selling companies things for $XX,XXX - $XXX,XXX is that you aren't doing the greasy used car / insurance sell. A lot of enterprise is project management with multiple stakeholders. Permanent recruiting is a similar situation.