It can be a relatively lucrative and low-risk (depends how you're treating it) way of making some money from the coins you already hold. The more established casinos that accept investments are also less likely to run off with your money, so the risk with plain theft is relatively low, though there's always a chance that your investment could be wiped out if a whale wins a large amount of crypto on whatever site you've invested on.

You could also just put your coins into something like BlockFi for interest on your coins as well; I believe the site's offering up to 8.6% annual compounded APR at the moment for investments under 5 BTC or something like that, and it's potentially lower risk than investing in a bankroll.