I've found that sometimes exist cases when you wish to modify only one method of class or only separate part of code and not distract on anything other in the object / file which is modified.
This feature should have a lot of common with "Reader" in Safari browser, when you can read article separately in pop-up.
The main aim is to give possibility to modify part of code (selected area, function, method) in the almost the same separate window, tab, pop-up. But from the other hand, editor should know that it's not a file, it's part of it in fact.
As the result you will able concentrate on the required part of code and make best you can ; )
What do you say guys?