Why should we break network architecture down into a number of layers

Imagine saving a word document to a file server.

The file goes through all the network layers to the file server.

If it did not the authors of word would have had to have written an entire protocol for transfering a file to a server.

The authors of the FTP protocol did not have to invent IP or ethernet for their app to work.

Its about not reinventing the wheel - u only code the bid u need to - & maintaining interoperability with other vendors.

we do it because it breaks the problem down into manageable chunks and allows us to reuse usefull functionality.

and by incorporating a well designed standard layer in the middle of the stack (IP) we can set things up so that the application developer has no need to know about the networks thier application is running over. This is what makes the internet possible.

Why should not we?
Because, imagine you have two companies, and the boss of the first company wants to send a message to the boss of the second company, if we do not have layers, like workers, cars, post boxes, then the boss has to walk down the whole street, knock on the door and go to see the other boss to say what he wants.
Well i think that is as simple as it can get