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

While the bus is moving, the luggage tends to remain in inertia state. When the bus stops, the luggage tends to resist the change and due to inertia of motion it moves forward and may fall off. That is the reason it is advised to tie any luggage kept on the roof of a bus with a rope.

Note: Inertia is the tendency of a body to continue in its state of rest or uniform motion unless or until compelled by an external force.