Paying for World of Warcraft allows Blizzard Entertainment to continue to create good content. It also pays for the salaries of all the employees (including game masters, forum mods, and server techs), pays to keep the lights on and the servers up, and gives the Blizzard bosses some money to keep them happy. Creating content, and keeping the lights and servers up is not cheap, and not free, so you have to pay to play.
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If you are using the trial account, then no you do not have to pay to continue playing the game. However, you will NOT be able to play the full version of the game until you pay for it.