Adult sites not using LPs

I want to do a PPV campaign where it would be beneficial to use the site itself instead of a landing page, but everybody on here says to make an LP for PPV traffic. It also seems ideal to not use an LP cause then I assume the viewer could just expand the popup window rather than clicking to open yet another window going to the site.

I notice that almost all adult sites that use popup ads use the site itself without making a special LP for the popup window. So why is it so important to make an LP for PPV traffic? I don't want to make any mistakes here because this will be a pretty big campaign.

By the way, I'm talking about LeadImpact. Somebody please hook me up with some knowledge here, I know some of you guys have a lot of PPV experience, thanks.