Yes, APFS enables shadow cloning of files, or Sparse files with a new copy-on-write metadata scheme. E.g. say you have a photo in your camera roll and open that same photo in another app, on the old HFS system the OS would make a duplicate file of that photo for the other app but with APFS that is not required. As long as there are no edits done to the original photo there is only one copy of it taking up space.
Expand this type of feature across the whole OS and you'll start seeing performance benefits and space reduction across the board with files, apps, updates etc.
I hope they make it so if I either:
A) have a photo in my library and I send it to someone via text message, it just places the photo in the message as a hard link to the original. But if I delete the photo from my library, it smartly just moves the original into the messages database to replace the link.
B) If someone sends me a photo and I save it to my library it will move the photo into the library and replace it in the messages database as a link.
Basically make it smart enough to never have that same image file in multiple places. And by always preferring to keep the original in the library when it's in both places, it'll make the messages database much smaller.
Maybe they could just make a smart photo album called "Photos in Messages" that just lists and contains all those photos (All messaged photos would automatically just go into your Photos Library and the message thread itself would just link back to it.) that's set up with sub folders for each message thread. But this album would be contained only on your personal devices and any photos from messages would only become part of the main library if you move them out of the album at which point they would appear in your photo stream and could be shared if needed. But Messages would retain a link to the photo (So it appears in the thread) until you delete the photo from the library completely.