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The premium members get their own benefits and rewards which the normal users don't get.
At the same time the premium members also have to pay their fees to get those premium services.
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Becoming a VIP means you are wagering huge amounts of money... and when you are doing that you became a valuable customer! Of course, the online or offline casinos will try to keep those customers!
I wish to focus on something else you said: "premium members get their own benefits and rewards"... That cost money! So online casinos can give you some money for playing, a special table for you, girl/girls dancing around, free drinks... and all that cost money! Offline casinos can offer you something else... cash/loss back, or whatever back there is...some crazy bonuses with low wager requirement...
Basically for VIP casino has to spend money! How much money they are willing to spend on this depends on casino management! Some casinos are willing to spend more to attract high rollers, others don't care a lot about that! There're many different examples around!
All in all, it's nice when your wagering can bring you benefits in the short or long run!