The shift + "Z" shortcut, as stated by the Google Drive Help webpage, says that this key combination allows the user to, "Add a selected item to an additional folder". This explanation wasn't the clearest to me, so let me explain it in a way that made more sense to me. Shift + "Z" allows the user to take a Google Drive file and place it in multiple different folders.
The reason why I think this shortcut is so great is that instead of making multiple copies of the same file and dropping a different copy of the same file in each folder, I can have the same file dropped in multiple folders. What this means, is that if I make a change to the one file it will appear that way in every folder. Instead of trying to remember which files I've updated, I can update one and know that it will show up that way in all locations. I use this all the time. If I'm working on a new presentation for a conference, I create a new folder with the name of the conference on it. Sometimes I'll use the same presentation at multiple conferences. Instead of making a copy of the presentation for each conference folder, I'll just save the same file in each folder.
Here is a quick video on how this works: