Question

give reason why does the fragrance of aggarbati spread in the entire room

Answer 1

its simple ...diffusion of gases

Answer 2

This is basically because of the process of DIFFUSION in which any substance gets passed on with the help of air.

When an aggarbati(Incense stick) is left in room, the smell of it spreads in the room as the particles of the air and the smell hit each other and it gets transferred and the room is full of frangrance.
This can be even done with the help of a lit aggarbati and the observation will be like it spreads faster.
Again for this you can take the property of DIFFUSION, that is The particles of matter spread faster when the temperature is high!