Underlying "Why is crypto so volatile?" is one fact: a coin's price keeps moving while it sits in an account. A balance held in coins changes value even when no bet is placed. A win can shrink while it waits for withdrawal, and a losing session can look better or worse than it really was. How a casino displays and converts a crypto balance decides how much of that movement reaches the player, and stablecoins remove most of it.
Keeping a simple record of each deposit and withdrawal, with the coin amount and its value on the day, is the only reliable way to know how a period of play really went. Coins left in a casino account between sessions carry the price risk and the account risk at once. Withdrawing what is not needed for the next session, and moving it to a wallet or converting it, keeps a casino balance from becoming an accidental investment.
Stablecoins are coins designed to hold a fixed value against a regular currency, and many casinos accept them alongside more volatile coins. A balance kept in a stablecoin keeps its everyday value from one day to the next, which makes budgets and results easy to read. The peg depends on the issuer and has slipped on rare occasions, and some stablecoins exist on several networks, so the network named at the cashier still has to match.
Either, and the terms name the unit; a bonus counted in coins moves with the market over a long wagering period.
Yes, when a balance is converted in both directions the casino's rate applies a second time, so the spread is charged twice.
The small gap between the casino's rate and the market price is charged at deposit and again at withdrawal, and on frequent small deposits it can exceed network fees.
Some casinos keep it in the coin, so its everyday value moves with the market, while others convert deposits on arrival and freeze that value until withdrawal.
A stablecoin is designed to hold a fixed value against ordinary money, so the balance keeps its everyday worth from one day to the next.
Noting the coin amount and its value when it moved is the only reliable way to see how a period of play really went.