Why would people buy, for example, $4.99 to watch Cars 2? Why not rent from Redbox for $1.10? Still trying to figure out the best way to use my R-XS..
The point of Amazon Instant Video is to watch content without having to go to a Redbox kiosk. Though I have a Redbox account, I rarely ever rent anything from them. While the cost-per-movie is cheaper, the convenience of Amazon Instant Video is, to me, worth the cost difference. It might not be worth the difference for you.
With Amazon, I don't have to get on my computer or grab my smartphone and find a movie I want to see, and figure out which kiosk I want to drive to. I can grab my Roku remote, find a movie, and watch it without leaving the house. It costs more. Most conveniences do. Again, as I said, it depends on if it's worth it. To me, yes. To you, perhaps not.
It is nice to have a choice.