It's a simple concept, offer a game for free to attract new players then do two things: Initially, the appeal was obvious for freemium style games. Ever since the likes of Candy Crush burst onto our mobile devices the UK population has had a thirst for gamified and freemium style games that has never really subsided.
You don't need to be involved in the online casino industry to know that the whole world is going gamification crazy and especially so at new casino sites.