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So far this is what I have figured out...

Coins - Plant food
Keys - Upgrades/unlock of extra levels that would otherwise be paid
Stars - The next world

See, I was hoping there was an in-game store that used coins instead of ream money. ******* YOU EA!
 
I was actually excited for a second when they told me about a store icon, thinking I could finally spend my coins, but nope. Just brought me to the plants page of the iAP store.
 
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