Really it does come down to marketing though I'm not sure everyone is effective at it nor do they have the same goals. For some it's simply marketing to increase the number of calls one does and for others it's more about brand management.
Many of the girls on here are marketing simply to increase calls and raise awareness of themselves. That's perfectly fine and one should do that if running a business, investing your time in advertising is just as effective as investing your dollars.
I know for myself and I see it in a few other active members it's more about Branding. I have an agency to do the advertising and bookings (though some take the task on themselves) and most any girl with decent photos and service with an agency can and will be busy, that's the agencies job. Rather for me I post on here with a not so easy goal of managing my brand. I recognize that my services are not for everyone, the experience I offer isn't necessarily the same as some other ladies (are two ever really the same?). Offering a glimpse of who I am allows the clients considering seeing me to make a better informed decision which is important to me. I would rather have one call that both the client and I feel was exceptional than 4 calls that are so-so experiences. It's not an effective use of my time and it's not only a waste of their time but also their money.