To expand on this: virtually everything is paid for by in game currency you can earn by playing games. Every game you play makes you money (you earn more for longer games). Using that money, you can buy almost everything.
The equipment you can buy with the in game currency gives you a real advantage in player vs player battles. Having no equipment at all means you need a very large skill advantage over the other players to be able to win games.
To get all things you might reasonably want to buy, you need to play an uncomfortable amount of games - which gets many people to take a shortcut and buy the in game currency with real money instead of obtaining it by playing games.
The equipment you can buy with the in game currency gives you a real advantage in player vs player battles. Having no equipment at all means you need a very large skill advantage over the other players to be able to win games.
To get all things you might reasonably want to buy, you need to play an uncomfortable amount of games - which gets many people to take a shortcut and buy the in game currency with real money instead of obtaining it by playing games.