What are free spins at instant bitcoin withdrawal casino sites?

I'm curious to understand the concept of 'free spins' at casinos that offer instant Bitcoin withdrawal. Could you elaborate on what they are and how they work? Are these spins offered as a promotional incentive to attract new players? Do they have any wagering requirements or limitations attached? Additionally, are these spins redeemable for real money, and if so, how do they impact the withdrawal process when using bitcoins?