How much money would you waste if Amazon didn't show you that you already purchased an item?
Far too often when it comes to blu-rays or video games, I see something on sale and think to myself "that's a good price, I should get it". Then once I click on it, I see the little banner on the top say "You purchased this item on..." and I think to myself "I was literally just about to buy that!"
It's a small detail I appreciate. And obviously it's usually just smaller items I forget about, but it's still appreciated that they do this. Because the reality is that if I bought second copy of a movie for $5, I'd likely never go through the trouble of trying to return it. And even though it would leave me annoyed with the retailer, I'd know the responsibility should end with myself.