Depending on your opinion, they offer you the option of either playing with your money or playing a demo game for fun. I think it's a good option to see what the game is like. This is a marketing tactic, making players feel more comfortable. They'll play a demo game for a bit before playing for real money, get comfortable, and then, when they're ready, they'll invest their normal money and continue playing.
At the very least, they'll see how it works beforehand.
They know it will work because it has worked before. They probably make the demo look as if you can actually win and let you enjoy that thrill until you get so invested to want to play with your own money. Once you do, you don’t win as much as you did during the demo games.
I agree, I've tried it before and it's true that I seemed to win more often in demo mode than when playing with real money. That’s exactly why I set a strict limit on how much I’m willing to spend when gambling, I never fully trust 100% in something like that. When the money runs out, switching to demo mode is a great way to keep having fun without risking anything.