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Amazon has lowered the price of Homefront and Divinity II to match the current Steam Weekend deal price. Homefront uses Steamworks so it requires Steam.
The only reason to buy Homefront from Amazon over Steam would be 1) the sale at Steam is over before you can take advantage 2) You are overseas and go through a proxy site to buy the game to bypass local prices.
The reason for Divinity II would be because the game comes with SecuRom and you don’t want to have Steam DRM and SecuRom DRM on the same game.