To earn money is an arduous and time-intensive task. It involves knowing where the best farmable goods are, ranging from creatures who drop sufficient magical items and loot at sufficient rates, to farming dungeons for sellable goods. While there are many fun and exciting ways to spend wow money, there are only boring ways of getting it.
Getting to the point where you can enjoy the game freely, without the hassle of worrying about money is perhaps one of the greatest freedoms a player can achieve. Imagine having the option to freely change your specialization any time you want, without a measly 50gp holding you back. Buy yourself your epic mount, never worry about armor repair and keep yourself constantly stocked on otherwise expensive consumables like potions and flasks.