Question

what are the benefits using layers in flash movie

Answer 1

Other applications will be able to reuse the functionality exposed by your layers.
You will be able to distribute your layers over multiple physical tiers. This can make a very good impact on your application by
improving performance (sometimes), scalability and fault tolerance.
The maintenance of your application is easier because of the low coupling between layers.
Adding more functionality to your application is made easier.
Layers make your application more testable.
Building a well formed layers makes the orientation in your application more easier.
Having your application not layered means that you have to deal with all security threats in one place which is very difficult. Having your application distributed to layers makes it much easier for design and implement
Without a good deployment plan it is not trivial to distribute your layers over multiple physical tiers in distributed computing. You need to plan ahead your layers when you create a distributed application.