Casino owners are already millionaires, but after releasing their tokens, they are aiming to become billionaires.

Yes, you're right about that. We can only hope for a generous airdrop for the users of this forum.
I would not say that the casino's own token will bring extreme profits through its value alone. It is more like a method of rewarding loyal users, and such things exist outside the crypto ecosystem.
It brings more profit as a feature, and it's a real wonder that not all casinos have implemented it. Obviously, whoever manages Whale.io, understands the business in its entirety.
There are certainly other advantages to issuing your own token, above all in terms of improving customer loyalty. For example, you can allow the token to be staked or unlock additional features when people use the token (e.g., win back part of the stake if a bet is lost, etc.). This creates a natural demand for the token and people will use it.
This also allows you to reach buyer groups who would otherwise be reluctant to deposit BTC and co. on an exchange. They can buy the token on other exchanges and make deposits that way.