I do it when I want to separate out the thinking/understanding from the doing. I might want to mull over how to do something instead of jumping straight into coding, and I find train journeys an excellent opportunity for this. Or sometimes I want to assess dependencies, perhaps I want to choose a library for doing X, I can sit and read through them on my phone and at least get a feel for them and whittle the list down.

I've looked at libraries on github during the commute from links in newsletters.

But never done any serious work off a phone, bar irl device testing for "hand feel" of the UI/UX