I've yet to find a portable screen and keyboard that can even remotely compare in display quality or battery life to a MacBook Air, and the 'portable' combination is usually heavier and always clunkier to set up.

If I'm doing that, why wouldn't I just bring my laptop

Single place for all your data, easier to manage and back up. Single device to take with you.

I don't personally think there's any ease of management advantage to be gained, but that depends on whether one wants their phone to have the data that's on their computer. I don't, personally. But I think there's definitely no advantage from a "single device" when you have to take something just as big as taking a laptop anyway.

I'm not trying to shit on this as a neat tool, but my phone is not the single place for all my data, and I find it considerably harder to manage. If I'm bringing my phone, an external keyboard, and a portable screen (and, presumably, a mouse) with me, it's not really a "single device", it's more like 4 devices that poorly imitate a laptop and don't offer as much functionality

Plus, when far from a homebase, some people are going to bring two laptops anyway so they can have twice the reliability of your average person.

Don't ask me how I know . . .

I've comfortably used a 7 inch tablet as a pocketable phone, such a thing with a keyboard would likely be a "complete" solution