Why it matters: Lighting when stopped

An unlit car with a breakdown on the hard shoulder along the A28 near Dwingeloo is almost invisible at night to oncoming traffic doing 100 km/h — the sidelight duty exists precisely to reduce that collision risk. Inside the built-up area the risk is smaller (lower speed, more street lighting), hence the difference in status — not because the rule suddenly disappears, but because the book deliberately makes it 'advisable' rather than 'required' there.