The jobs on the job boards are the ones that cannot be filled by referrals, career pages, etc...It's really a last resort.
If you post a job to a semi popular job board, you get a glut of CVs, most of which are completely irrelevant. You hire a recruiter to protect you from that.
I can understand the case for hiring a recruiter for a small business (what I would consider less than 50 employees), where a HR department might not be a permanent fixture. But when I see recruiters for Fortune 500/50 etc companies, I do wonder what's going on there.
If you post a job to a semi popular job board, you get a glut of CVs, most of which are completely irrelevant. You hire a recruiter to protect you from that.