I guess this requirement is in every casino and some may even ask to wager the deposit twice or thrice before you can withdraw. There are two main purposes of it. First, when people wager, they will most probably lose money and hence it is a profit for the casino.
Secondly, the other reason that the gambling sites will give publicly is that this helps prevent money laundering and other illegal activities.
Why would you deposit on a casino site when you do not want to wager and gamble with it? If this wagering requirement is lifted, some people may use these platforms for money laundering, that is depositing on the gambling sites and withdrawing at the other end without gambling.