My gf used one. She had a legal right to quit but it was inconvenient for the company. They refused multiple times, they also gaslit with "what about the children we teach, if you leave the school might have to close" etc. etc.
Using the agency means you do a 10 minute phone call and that's it. You don't even have to work your remaining days or talk to the company ever again. The agencies seem to have some legal powers that a normal person doesn't, or at least in reality they get results much faster and aren't allowed to screw around.
What did amuse me is there's a discount if you use them multiple times.