The benefits of using a staffing agency as a company is that it takes a lot of the burden and expense of searching for qualified candidates away from the company, and places all the recruitment, screening, and testing responsibility onto the agency. It also allows companies to “try out” new employees without fully committing to them--i.e. onboarding with benefits and government tax paperwork. In industries where there’s a lot of turnover, or where needs fluctuate, companies appreciate working with staffing agencies that can fill many spots with qualified part-time talent for as long as the company needs them and no longer. In simple terms, a staffing agency reduces the “cost per hire” and protects companies from having to pay unemployment on employees that weren’t around for very long.
When it comes to using a recruiter-type of specialized agency, the benefit is nearly opposite in that these agencies devote time to finding the best passive candidates (i.e. those who are not looking to switch jobs) and convincing them to move the client company.