I'll tell you why some people use a case. One day, a guy decides to show his nice, expensive retina Macbook Pro off to his friends. They're all impressed with the screen, oogling over it. One of them says...
"Hey, Bob! You gotta see this"!
"Alright! Scoot it over to me"!
Guy then proceeds to push your Macbook across some gritty table, making horrible scratching noises in the process...
Which forces you to make a face like this:
So that's why most people use cases. It both protects their expensive stuff, and makes it easier to carry around.