Editing on phone. Phone dies. Scrounge up tablet, edit again, only rethink some of the work. Hit save. Plug in cellphone. Cellphone turns on. Now what?

Depends on the granularity of updates. Did the last changes get sent immediately? Are they gated by a save button? Are they periodically pushed?

Some of those don’t need a durable undo, but the rest definitely benefit, and undo has other uses.