I run a forum dedicated to a freemium app on both Android and ios. Its a pretty popular game which I think is down to it being free!
If you want to rush through the game, then you can spend money, however a lot of players like to take their time and complete it for free.
I probably would never have downloaded it and got addicted to it if it was a paid app, its amazing how they draw you in and get you to spend money in the end!