Different colors and different materials. I have a Space Grey Aluminum that looks black, a silver Stainless Steel, and a white Ceramic Edition. All look different, and all match differently with different colors and styles of bands. I rotate between the three watches every day, and change bands every day.
Pre-Apple watch, I had multiple mechanical watches as well...different colors and materials. I have a bunch of different pairs of Nikes, too, even though they're all....sneakers. Do you have exactly one of every item you enjoy wearing?