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

Explanation

A luggage is usually tied with a rope in the roof of buses. When a moving bus suddenly stops, the luggage on its roof remains in the state of motion due to inertia of motion. Hence the luggage falls down from the roof of the bus. Similarly, when a bus suddenly starts, the luggage on the roof of the bus continue to remain in the state of rest and hence falls down from the roof of the bus. Thus, the avoid the falling of the luggage, it is tied with a rope on the roof of a bus.