What makes you want to buy a membership at a GPT site?

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As I am launching a GPT site in open-beta early next month, while working on this site I came up with a certain "question" numerous times in my mind so I felt like finally asking it. What makes you want to buy a membership at a GPT site? Do you prefer the referral %'s you can get? the extra "pay per click", other unique features? Possibly even the loads of referrals you'll get by getting one? What are the reasons that you if you're one to buy a membership, are?