In a way, gifts aren't rational things anyway -- they are emotional ones, which is why any rational response you get is going to sound cold and calculating (eg., better service in the future, etc.). People give gifts because they want to. Otherwise, why should people exchange gifts at all at Christmas time? They should just tell their friends, "Look, I'm going to buy you something you don't want, you're going to buy me something I don't want, why don't we both just save our money and use it on ourselves?" Makes sense, sure. Is it fun? No.
But just to explore your question for the sake of exploring it, I would say SPs by and large deal in the pleasure business, which is why they elicit a more emotional response than say, getting your car fixed or getting your teeth cleaned. An orgasm by definition is an emotional thing.
If prostitution is really the oldest industry in the world, I would imagine so is tipping your favourite girl. Some of it has to do with making a girl feel good who made you feel good, some of it is human nature in wanting to be thought of as special (ie., be her favourite client), some more of it is male competitiveness (ie., getting your favourite girl the biggest and bestest present ever), and some of it is hoping for or expecting special treatment in return. These are the same feelings and expections people have in their other relationships, just with the volume dialed up because of the sexual component.
Gifts only make sense if you think of people as emotional creatures and not rational ones. Then it makes all the sense in the world.
