The short version is that Hearthstone uses just a single currency for both paying and free players, but your progress at earning that currency drops off sharply if you’ve already done your new quest for the day. However, you still have the option of playing, earning, and advancing via ranked play despite that restriction, so purchases feel more convenient and satisfying without being mandatory. Check out the full article if you can’t get enough examination of the mobile free-to-play model and want a close analysis.
Hearthstone is a successful title on phones and tablets. It’s successful all around. And yet if you look within the field of mobile free-to-play titles, it breaks a lot of the “accepted” rules about how you have to monetize these titles. An article on Gamasutra takes the time to analyze how its different assumptions about progression and rewards produce a different game environment that generally feels more welcoming and rewarding for players.