so you have to craft cards. how? from dust. how i get dust? from disenchanting cards. how i get cards which i can disenchant? you need to buy it = money
the above thing is an example, where money comes into play. and ofc this is not general thing (but players do it), and because people are lazy they dont want to grind that money day by day to buy the 100 gold pack (F2P) so they will rush into store and buy packs for real money