I think this site is ignoring the value add for the interviewers as well. Interviewing others is a skill and takes practice and dedication. Many people just take some coding question and ask a candidate without thinking it through much and what they're actually analyzing. It would be good practice for interviewers as well.

You mean like: they could allow a third person to watch/listen the interview and at the end of it, rate the interviewer and give suggestions for improvement?

The intellectual challenge is a baited hook for headhunting people who aren't looking for work, so aren't actively applying for jobs or answering cold calls.

I can see it as good for practicing for the interviewer. It'd also be good if I could easily make an offer out and use that as a way of sourcing candidates and getting my referral bonus on.

States on the page that it is possible to hire people that you interview anonymously. So it could be a good source of people.

Every interview you run has positive expected value - you get better at the process, and it may end up in a positive-value hire. If you've got low opportunity-cost time, this is a great tool for consuming it.

Interviewer can be the person responsible for hiring in a corporation/startup. He/She can hire the interviewee. This is a win win!