Many games incorporate micro transactions into their games, but few do it well. Many players feel cheated by the fact that people can "pay to win", or that the game is centered around having to "pay to win". By creating a system that is free to play, but doesn't require the player to have to spend money on the game will increase the player base growth of the game. If players feel like they can reasonably grind the in-game features that players who pay also receive, then they will stay satisfied and continue playing the game, as well as bring in their friends.
Example: Hearthstone: Heroes of Warcraft
In Hearthstone, the player receives daily quests that allow players to gain in-game currency (gold). This system allows players to gain gold reasonably well, which allows them to progress well without having to spend a dime. Players are also able to use their in-game currency to buy the games expansions, which few other games allow.