Staffing agencies, from a super cynical point of view, take a slice of money that would otherwise go to the people actually working.

I think in the more nuanced view staffing agencies allow people with irregular rhythms to still have a job. And of course helping people actually get a foot in the door! But if you're going to work full time anyways, and there's more open-ness to you being hired, why go through a staffing agency? I think the reasons end up being limited to "could not get hired yourself".