I can definitely see cart remarketing as creepy and appreciate the Walmart example.
However, in a virtual example, someone could be shopping Walmart, Target, Kohls, Sears, etc. at the same time. Getting ideas, filling up their shopping carts (so they don't forget what they saw), comparing shopping carts to other stores to find a deal, taking into account any shipping considerations..... and oh.... I am only on lunch break or have to get ready to make dinner.
So... I'll come back to this later...
Then... like magic.... "you've got mail". Inside is the shopping cart you were putting together (that you thought was all but lost). And, even better, there's a 10% off offer if you make a purchase within 12 hours.
As price is a big factor in why we buy it gives a potential customer one more reason to stop by... just to be sure. ;-)
The company I work for, Listrak, has some pretty impressive case studies to show how profitable rescuing abandoned carts can be. For vendors... and customers! Shopping Cart Abandonment Case Study