They make people feel they own something "special".
Loyalty programs, gift cards, cashback, airline miles, and gaming currencies all encourage mental accounting.
They make people feel they own something "special," increasing engagement and making them less likely to leave the platform.
Why does Google have no such GC's, because they have the power of users. Ultimately it ends at company profit, we are a small part of it. So don't be overwhelmed with GC's and all concepts like this.
Thank you for your attention.