1) They already have a client base and want to make them aware that they are available.
(I don't always email that I am coming into town and I will put up an advertisement)

2) Its another stream of income for the provider (diversify)

3) They may feel that if you are willing to pay you are not the police thus a more comfortability for setting an appt.

4) Its a scam and you will get pics of Gisele of some other model and they have your $$$

Em

Emma makes some good points.  I think some use it as a way to pay for the website domain as well.  I personally find it to be distasteful.  If they are smart enough to have other photos that you may view I am okay with it but when all you have is a "pay and see" portfolio then you lose my interest immediately.  But, to each his/her on!

being too easily known or too widely distributed. Not that it's by any means leak-proof, just a hindrance. It doesn't bother me much, as long there are enough free-to-view photos to allow me to assess her appearance. If there aren't, I'll skip her, regardless of whether she also has a "premium" gallery.

In my opinion, a less questionable approach is to make a gallery accessible only with a password she gives out to her established clients.

escortsites websites

I have noticed that even the escort site website I have, the company that owns the hosting and design of all these sites are posting all kinds of money making schemes on all the ladies sites as a possible money generator.  I think they get a percentage of the money. I'm not sure i'm looking into it.
My client’s corner site popped up with a premium photo area and I think you have to become a member and pay to see but I didn’t put this up  the company did. I am trying to figure out how to take it down because it’s only taken photos out of the other galleries I have and pasting them in it.
So be careful with the ones you chose to pay for and ask the lady if it even has different photos and make sure you haven’t seen them.
just my 2.0

see if its the same thing you are finding on others. This is the clients corner i have kept with escort sites... http://elitegfeconnections.com

my two cents

the site took photos from my regular gallery and put some in premium, regular but sexy photos.... I cannot even access them! Fans of my site are wanting me to email them pics. I am not even seeing any of the revenue from this "PREMIUM" section even though you  gentlemen are paying. I have tried contacting the site because its not fair to you and I personally dont want to put out pics like that, leaves more to the imagination which I think is sexier...