Actually once you 'buy' the spellbook you can easily open 5 chests per season. Which unless you are very unlucky they will actually 'earn' you cards or DEC.
So yeah that 'play to earn'.
But if you want to earn more then sure, go ahead and level up some cards are move into higher leagues.
So what it actually is it's "Pay to Play and Earn More"
GLHF