Why would an employer need to use a job agency to get employees?

Every place I worked at, no matter how professional or in what field, had 100s of resumes to choose from.

My aunt works at a doctors office. She even has resumes of a few MDs at her finger tips.

Why would an employer need to use a staffing agency? I can literally throw up a fake job add on Craigslist, and get 10 resumes before this night ends.

Is there something I'm missing here? Why spend a bunch of cash using an agency, when I can save cash just and hire directly?

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