"No new content to save" message

Im working on preparing a bunch of animals for game play. ATM, everything i under the same directory. I was looking to break it out into different sub directories (animations, textures, etc). Since I have 20+ animals, the original idea was to go through and create the directories in each folder and then go BACK and move the files as needed. The folders got created, i hit save and nothing happened. I exited the program and went back in, the folders were NOT in UE4. I went back, created the folders and put something it in, hit save and got the normal save message, exited, came back in and everything was there.

First off, why cant u save an empty folder? Secondly, if u CANT save an empty folder, is there a way u can detect one and put a message up saying “empty folders wont be saved”?