Some programs like LibreOffice or Microsoft Word provide a way to open a second window (or even a third one) to an already open document. The function in question is located there at Window>New Window.
These programs let one open two or more views into the same document at different places, e.g. to compare what is said at one place with what is said at the other one. You can easily switch between different parts of a large document and manage several "working places" without loosing focus by opening independent windows at the places where you work or check things.
Of course editing is possible in multiple such instances. You can e.g. correct a typo in any window open in the same document. If views overlap with respect to the document, you see the changes happening in parallel to the other views as well.
For Mathematica a use case would be to independently scroll through some code and the output which it has generated – but many pages below from where it actually is programmed (e.g. if the code is armed with some conditional print statements which lets you check the flow of control).