Pay-To-Win games play off of the premise that the game is free, and you can still play the game, but if you pay us real money you can get something that will allow you to be better faster.
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Pay-to-win appeals to the casual gamer. You pay a bit of cash and you're automatically better off. You don't have to play for actual days to get the items/levels/whatever and your game experience is improved. What would have taken hours of actual gameplay has been just given to you. You just paid to win, and now you can play the game as though you had just spent hours playing the game.
But it's not all good. The Play-To-Win style of gaming cheapens the experience, for many gamers a pay-to-win system ruins the whole gaming immersion. No longer is the game contained within itself, the real world has leaked in and changed the entire game.