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

When some luggage is placed on the roof of a bus which is initially at rest, the acceleration of the bus in the forward direction will exert a force in the backward direction on the luggage. In a similar manner, when a bus which is initially in a state of motion suddenly comes to rest due to the application of brakes, a force in the forward direction is exerted on the luggage. This can cause luggage to fall from roof.