Scientific research by scholars in the field of Economics of Prostitution sees the role of escort agencies as screening "high quality", i.e. disease free, prostitutes, from those who are low quality but trying to pass themselves off as high quality. All this in a market where a certain segment of the customers are willing to pay a premium to assure themselves of seeing only highquality workers.
Please see the paper: Prostititues, pimps, and brothels: intermediaries, information, and market structure in prostitution markets, by Amy Farmer and Andrew Horowitz, Southern Economic Journal, Jan 2013
