Free is great. But not free can accomplish two things.
1 of course is providing revenue for the site. Obviously, it costs something in servers, bandwidth, etc. to run the site, even if you're making no ongoing improvements. If you don't charge at least some user, you have to make revenue via alternate means, such as advertising or selling contact lists.
2 is, less obviously, improving the quality of job listings. Craigslist started charging a nominal fee about a year and a half ago, I think, just to ensure that job listings were accurate -- after all, who will pay even the $25 CL charges to post a listing that they aren't sure they'll hire for?
A free site is indeed feasible, but how do you accomplish these two things without charging? This is the crux of your business model.
What is their incentive then for providing the service?
It is a typical "middle-man" service/business model.
I tried creating a free website for students and recent grads - jobsphere.org - but i have put it on hold - its not up - because it would take too much time to get people to join and companies to join.
I'm playing with the idea of just creating a site with the direct career pages of companies - since essentially what 90% of the job boards out there do - just run scripts/spiders to grab the public info and repackage.