Bitcoin casinos just work as any other casino, you need to play with money - which is usually bitcoins here - to play the games and have a chance of winning money. A bitcoin casinos doesn't necessarily mean that you need to pay only in bitcoin, it is just that they accept bitcoins as a form of deposit and withdrawl. For many of us here this is very attractive, so we don't need to convert cryptos into FIAT and don't have to pay fees on it.

I am not quite sure what you mean by next rool game predicitions, Is it maybe a game based on bitcoin blockchain? There are sometimes raffle games on the forum that use the next bitcoin block to determine the winner. You can choose the number or latter at the end of the next block.