I clearly remember these folks. They acted in almost exactly the same way as literary or artist agents; searching out opportunities for their clients, and setting up interviews, etc. As a hiring manager, I dealt with them frequently, and had friends that used them.
They used to make a lot of money, because they would charge a percentage of the rate they negotiated for you.
Nowadays, it looks like they have been replaced by "race to the bottom" sites, like Upwork, or these contract companies, that hire you at a fairly low rate, and shop you out for very high rates. You get to "enjoy" the crappy treatment most companies give to contractors, but at rates lower than the employees that sit next to you, shooting spitballs at you.
I encountered this, when working with recruiters, after leaving my last company. The ones that didn't immediately hang up on me, after finding out I was older, started trying to lowball me into being one of their contract shop employees. They would love telling me that I shouldn't ask for too much, "because of my age," before "generously" mentioning that they happen to have a contract shop that would be willing to do me the huge favor of "throwing some work at me."
It's a real slime-pit, these days.