Ask HN: Hiring managers that use recruiting agencies, why?

Hiring managers who work with recruiting agencies: what sorts of resumes do you see come in through these recruiters? What are the benefits or potential advantages for a hiring manager to work with them?

I'm an engineer, and like most folks I tend to ignore recruiter emails, but I've received a couple recently that looked very interesting. I'm wondering if I'm letting a genuinely good opportunity slip away by failing to respond?

(Note: I'm specifically referring to 3rd-party agencies, not internal recruiters.)