There are two different casinos found throughout the lands of Dragon Quest 11, each casino with its own unique prizes.
Sometimes you'll be unlocking powerful new gear, while other times you'll be unlocking things like the bunny suit costume for Jade. Like real life, winning at the casino requires a bit of luck, but since this is a video game there are some tips and tricks that can be used to get the odds in your favor.
You'll want to spend time in the casino, then.