For a guy the benefit of an agency is you only need to screen once. You can see all the girls that come and go thru that agency with a simple phone call once you are screened. With an agency, someone will always pick up the phone. The girl you want to see may be busy when you call, but available in two hours. The agency can set your appointment while she has someone else's dick in her mouth, and give you the room number while she is brushing her teeth. Indi girls you need to book ahead and they sometimes cancel/change tour schedules for a variety of reasons.
For a girl, the agency does all the screening/booking. All the girl has to do is suck and fuck, and clean up in between. Some girls need to be with an agency because they don't have the business sense it takes, are naive of dangers that lurk, or cannot attract enough clients as an indi.
A lot of girls start agency, then go indi once they establish a client list and/or their own good reputation.