VIP level is simply just way of the casino to give back something from your wager amount since they guaranteed a profit whenever you bet more due to the house. Imagine getting 15% to 20% max rakeback while you lose most of the time your bankroll.
Basically, Casino doesn't give us money because they also want to raise money from the players unless we are lucky then we will get it and VIP Level is a way of casino to attract the attention of players in business competition, as a player we will choose a casino that offers higher. 15% of House Edge income (1-5%) is not a proper amount but it's better than nothing. In addition, 15% offered will be divided into several bonuses such as 5% for weekly bonuses and 10% for monthly bonuses or maybe casino uses a different formula for rakeback, weekly and monthly.