Some are quite simply criminals.
Some are just bad and need to be stopped or put right.
The agency the company I work for had been using, it turns out, had not been doing their paperwork.
We recruited one of the workers full-time. He was without any of the proper documentation. We had to help him get himself national insurance registration and prove to the home office that he was legally allowed to work in the UK amongst other things.
I did hear from the agency workers before about being paid late nearly every week.
My company paid the agency for them to sit on the money, earning interest, whilst leaving their employees in a position where it was difficult for them to get by without borrowing money off other people,. They also couldn't easily find work with more reputable employment firms as many knew little about the official paperwork they should have.
Some of them worked evenings and weekends doing second jobs just to pay the rent.
At least when they did work in my team, they were treated fairly and with respect, but even for these individuals, that's less than half the story.