I think this is well illustrated in a lot of science fiction. Irregular or abstract tasks are fairly efficiently articulated in speech, just like the ones you provided. Simpler, repetitive ones are not. Imagine having to ask your shower to turn itself on? Or your doors to open?
Contextualized to "web-apps," as you have; navigating a list maybe requires an interface. It would be fairly tedious to differentiate between, for example, the 30 pairs of pants your computer has shown you after you asked "help me buy some pants" without using a UI (ok maybe eye-tracking?).
Contextualized to "web-apps," as you have; navigating a list maybe requires an interface. It would be fairly tedious to differentiate between, for example, the 30 pairs of pants your computer has shown you after you asked "help me buy some pants" without using a UI (ok maybe eye-tracking?).