Changing agencies benefits the girl by keeping her employed for a while until her new agency also tires of her (or vice versa). Seeing that a girl has worked at so many different places, though, is kind of a red flag, especially when they go from a higher dollar agency to a lower dollar agency.
Changing names probably boosts bookings if the girl had gotten lousy reviews under the old name. Not everyone reads the changing names, changing places thread. If the girl changes her name and her level of service early enough in her career before she is widely circulated, she might be able to redeem herself. And some names are probably changed once a girl realizes she might be getting confused with a different girl with the same name at a different agency.