A lot of the best people a company wants to hire are the people who are happily employed, whose employer wants to keep them, and need to be talked into applying

A lot of the best people a company wants to hire are the people who are happily employed, whose employer wants to keep them, and need to be talked into applying - especially if a candidate might be in a different city and would need to relocate, etc. Even if someone is already applying for jobs, if your resume goes through a recruiter, they can talk you into at least interviewing at a place you otherwise might not, which gives the company a chance to sell themselves to you (for instance, there are a lot of extremely talented and exciting teams and projects in otherwise boring companies that you might not be aware of). A recruiter can often do some level of assessing candidates before their resume gets to an engineering manager, saving a LOT of time for someone who's already very busy. On some level, it's the idea of outsourcing things that someone else can do better. If you find someone who can bring in great candidates for interviews better than you can, why wouldn't you hire them, so you get better results and can focus on what you do better?