I think the "only indicate on pulling out if there's someone there to see it" thing is primarily an easy way for the examiner to see whether you're actually checking properly before pulling out. Which is something they want to be really sure of, on the whole. And is qualitatively different to turning at a junction because other drivers are (or should be) aware of your car as a potential threat at the junction because it's already moving, whereas telling the difference between a car about to pull out without indicating and a parked car is pretty hard from a moving vehicle.