I needed it for intensive Photoshop editing of large, RAW files. I have an early 2013 15" Retina MBP with 16GB RAM (which I'm returning to the store), and a late 2012 cMBP with 8GB RAM (which I'm also returning). In my estimation, the cMBP with 8GB of ram is much faster than the retina with 16GB. Don't ask me why.
Why are you returning both, and what are you going to replace them with?