They're not the same, after all :)
but to the main thrust of the question ... why would I want to "code on a portable projector" ... when I have a great screen on my laptop which I would otherwise be connecting to said "portable projector"?
If you want to talk about bringing your own showcase gear for demoing what you've done / teaching a class / etc because the location(s) you visit may not have one, that's a different story
But I never have thought of hooking up to any kind of a projector "for coding/text" if I am by myself