Blizzard implements all of these mobile cash designs into their games to keep players attached and unknown players that are new to the game think they need to spend some cash to reach the ingame goals like legend which is not the case.
These kind of monetisation models are applied everywhere in the game industrie because apparantly people have to much money to spend thinking they need many cards.
They are really sqeezing the reward system for a overpriced game where you need to pay a ridiculous amount of money for some cards to have a variety in gameplay.