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.
Source: Gamasutra