Because you have a greater chance of sustaining a majority of the traffic. TGPs dont provide the best porn in the world, just some links to some free stuff (i.e. 10 second videos).
Now, if you send SE traffic to a TGP, alot of the traffic will take an interest in it. Those who are cheap will merely bookmark the site for their free porn purposes. Those who are actually willing to spend money for porn, will realise the best porn is the stuff you pay for. They will soon get sick of the 10 second videos and eventually join a site - hopefully one of your sponsors.
If you merely send SE traffic to a paysite, you immediatly loose a heap of people. They just click right off - even the ones who might otherwise pay. By sending it to a TGP site you can sustain good traffic, which can be used to further yourself in the future.
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