Presumably he also offers the applicant some sort of stability ?

Without a middleman, the H1B holder is very tied to its employer and has a difficult time changing job or bouncing back after being laid off. A middleman could smooth that out, by keeping the employee a bit longer on the payroll until they find another contract. I know people using those kind of services in the UK.

Also if you are a permanent employee with good benefits, getting a quarter of what your company sell you for is not bad.

Anyway, that's a lot of "if", I agree.