Why is it advised to tie any luggage kept on the roof of a bus with a rope?

Explanation

(a) if the bus starts moving suddenly, thern due to its inertia of rest, the luggage kept on the roof of the bus tends to remain at rest and hence may fall down from the roof of the bus.
(b) if the moving bus stops suddenly, thenb due to its inertia of motion, the luggage kept on the roof of the bus tends to remain in motion and hence may fall down from the roof of a bus are tied with a rope, they cannot fall down when the bus starts suddenly or stops suddenly.