It's generally very difficult to win money from a no deposit bonus, but some players get lucky. The aim of the bonus is to allow players to try some of their games for free. They also have the chance to win some real money, which is a bit more thrilling than playing the games in demo mode.
Playing in demo mode won't allow you to win any real money. However, you can test the games without obligation, learn the rules if there are any, and see if you like them.