They set up the first interview. They are responsible for taking taxes out of your salary. They negotiate the deal with the client company for the first interview. You are never told the bill rate of the client company. You are never told what percentage of a cut you get.
Recruiters are a wily bunch.
After the fourth round, they realize that something is up, and they clam up tight. There are many different types of recruiters. At worst, they are buzzword hunters, who scour the internet for any resume with a phone number. They are telemarketers, working on commission, making 20 or so phone calls a day, telling each person that, yes, they work in your best interest. As soon as the contracting house finds out what they are doing, no good deed of theirs goes unpunished.