To keep the 'economy' under control. They don't want you to be able to afford all of the things in the store with free currency, they want you to spend real money on them, so they regulate the in-game coins to ensure you don't earn them too quickly.
If you are talking about Coins these are earned by doing Daily, weekly and monthly missions witch add up if always done.
You also gain bonus weekly gold by playing the game and earning XP for the alliance system.
If you try to do your challenges and missions when you are in a stronghold or free play, you get the best of both world
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