It is tough to win money using a no deposit bonus, but some players are fortunate. The goal of the signup casino bonus is to allow individuals to try some of their games for free while also winning actual money, which is a little more exciting than playing the games in demo mode since you can't win any money on a demo mode game.
Playing in demo mode will not let you win actual cash, but it will allow you to test the games without any obligation, learn the rules, and see if you like them.