I think it's 50-50 between encouraging social interaction and encouraging people to buy stuff. The Summer Sale is a good example because it would be a real challenge to get the level 5 summer badge without spending quite a bit of money in the Steam store. You only get about 1.5 badge worth of cards for free from voting. From there on it's from buying stuff.
Other game cards are often sold for TF2 keys or gift games or trough the marketplace. Either way Valve takes a cut. Pretty smart actually.
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The Oddworld Wiki
When one person suffers from a delusion it is called insanity. When many people suffer from a delusion it is called religion.
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