I don't understand what "files" means to you. If you have two multi-page documents, you have to somehow arrange them on the screen, and you need to show functio

I don't understand what "files" means to you. If you have two multi-page documents, you have to somehow arrange them on the screen, and you need to show functions to edit/view/[...] either of them. But, whatever you do, you need some kind of borders to understand where one document ends and the other begins, and you need some controls to change the view of each individual document (such as minimizing or maximizing it) and you need a button of some kind to close the document. Then, if we have two documents and floating bars that you can put wherever you want on the screen, it will be completely impossible to tell which bar controls which document (or displays the state of which document), so we should probably put the function bar somewhere in the document container. And, hey presto, we've reinvented the Window, because it turns out to be a really really useful and natural abstraction.