incalls but some others are essentially referrals.  Nevertheless, the fundamental public face is the agency.  Haven't seen one yet that doesn't have a web site, but may be.  While I have no direct knowledge, my impression is that the agency takes a percentage of the contribution.  For all practical, if not legal, purposes the providers are employees.  

There are screener/schedulers who work for a group of providers and who may even do ads - one was posted today by a leading screener.  However, as I perceive it, the screener is the employee and the providers employ the screener individually.  There is no brand identity for the screener, however well known.

There are groups of providers who share incall apartments.  I understand that there is usually one who has the lease and the others compensate her for the use.  To me this is simply cost sharing and fundamentally different from agency incalls.

To me it's sorta simple: the agency is the brand that guarantees the quality of the "product."  the providers may come and go, but it is the reputation of the agency that attracts the client over time.