I don't know why so many people are under the impression that they do thinness for thinness sake. They do thinness for
lightness sake.
The lighter your laptop (or tablet, or phone), the more you'll choose to not leave it behind but rather to just lug it around with you, and so the more places you'll have it and the more it'll be there to aid your productivity in random situations. [Also, for phones and tablets specifically, the longer you'll be willing to hold it up to your face to stare at it before putting it away due to the "gorilla arm" feeling.]
Sometimes I actually carry my MacBook around in my backpack when I'm just downtown for a meet-up and have no plans to do any work. I get it out for the same reasons you might pull out an external Bluetooth keyboard for a smartphone—e.g., if you want to type a long response to an email, or need to type a snippet of something that's awful to type on a phone keyboard, like code. Except that this Bluetooth keyboard happens to have its own computer attached to it.
(I don't use it at home, though; at home, I use a Hackintosh with a real keyboard. Which happens to be an Apple Magic Keyboard 2 with the exact same butterfly key-mech in it that the MacBook and rMBP have. But Magic Keyboard 2s aren't getting gummed up left and right, because the butterfly key-mech itself actually works fine when it's given adequate travel height. It makes a huge difference; you wouldn't even think it's the same key-mech!)