Because this deal makes Amazon something much more. It makes it a company that Comcast (CMCSA) and AT&T (T) need to worry about. It makes it a company that local TV stations, and the companies that own them, need to worry about.
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This deal makes Amazon a broadcaster.
Consider. Twitch is currently capable of delivering 1 million live streams at once. It has 45 million subscribers. Amazon is getting its technology as well as the programming. Amazon can use the cash flow generated by Twitch to extend its cloud presence and networking capabilities, not only to 1 million live streams but beyond. Netflix (NFLX) has been building its content delivery network on subscription revenue. Amazon can now follow it with ad revenue.
Amazon can turn a profit on Twitch by integrating its e-commerce capabilities into Twitch programming. Instead of having ads in the stream, Amazon lets Twitch offer commerce. The "sales funnel" on anything a Twitch viewer may buy suddenly narrows. See something you like? You can download it. Or you can order it for delivery tomorrow. This doesn't just mean games but a whole host of technology and gaming-related products. Gamers are big buyers of technology. Many will now be buying Amazon's.