Same reason writers tend to carry around a moleskine, or artists a sketchpad. Ideas don't always come when you're absolutely ready for them, and being able to get something out right then and there is a lot less frustrating than trying to remember that idea hours later when you're in front of a "proper" device.
I've had some long trips a couple times where I would have loved to pull out my nexus and do some web programming. The natural solution would be to get a Surface 3, but I've had a Nexus 10 for a while and really enjoyed it. Battery life lasts forever as well.