Whatever you invest in any gambling sites will be used as bankroll for that casino and whatever you may earn out of that investment depends upon the profit or loss site will make in future proportional to your investment size against total bankroll.

You invested 1BTC
Site have total bankroll of 100BTC
Site made 10BTC in profit in one month.
You will get 0.10BTC in profit with 1BTC investment in one month, after profit your new investment amount will be 1.10BTC

And your balance may also get deducted in same proportion if site loss 10BTC total in that month. So investing in gambling platform also have risk but that is quite lower than gambling. Return is very low while there is also risk of losing all you have invested.