I just wonder what would a gambling site do with those documents they require related to KYC.
They can keep it and store it as their evidence if something goes wrong. Just like other websites or establishments that are requiring some documents for their services but when they are done, nothing happens as its only stored to their database / vaults.
It would just make the process of payment (of winnings) longer.
As others explained, their government maybe requiring it or they are pushed to implement it and they just want to obey and protect their business.
For sure anyone would think twice of submitting their personal infos to those gambling platforms.
But if its your money to cash out, you have to follow.
It is because now a days most of the companies are auditing, so in this case they need to get clear idea about how much investment and through whom they are getting. While maintaining those it will safeguard for future transactions.