Editing XPS: why?
XPS is intended to be a fixed representation of a printed page. Which means the layout is fixed, the typesetting is fixed, it contains all the resources and information required to represent the document exactly like the author intended. So if the recipient opens the document the text won't reflow, the fonts stay exactly the same the layout will not change, etc... Really a good format for sharing and printing.
But why would you want to edit this? You can always go back to the original file, do the modification, publish back to XPS, and done!
Let me give you a first good reason: time.
It's a cliche, but it makes perfect economical sense: time is money. Or more precise, time costs money.
If you have an XPS file with a small text error in it, and you want to modify it by going back to the original file, you risk losing more time than really needed.
You could have the original file on your computer, and the original software you used to create the document - then you're lucky, it's the best case: you created the XPS file yourself, and you are using it right away, it is probably not so much of a hassle to do the modification and export it back to XPS.
But what if you didn't create the XPS file? If you are receiving this XPS to print, you need to contact the person that supplied you this file, ask him to do the modifications, re-export the XPS file and send you this one back. This takes time, by blocking your planning and by going back to the creator for a new file.
Same scenario if you are responsible for publishing the XPS file on the site. If you notice an error in the document, you need to go all the way back to the creator to ask to do the modifications and regenerate the XPS file, run through the required procedures, etc... This takes again more time than it should.
An application that allows you to do small, corrective edits on XPS files saves you time, and money. When you notice the error, either as printer or publisher, you fire up the editor, make a quick change, save, and continue your work without losing valuable time.
Case in point: an example.
Microsoft publishes an XPS document with the contents of Windows XP SP 3 here
On close inspection of this document, you see a spelling mistake on page 4 (the word 'Updaets' should be 'Updates'):
I'm not familiar with the procedures to get a document updated on the Microsoft site, but I'm sure firing up the NiXPS Editor and correcting this is a very time efficient way to do this, without having to go back to the original doc, re-exporting, and whatnot.