I can have multiple payloads that I can swap on the fly without having to organize with a bunch of duckies. I can improvise based on the needs of the operation. In theory, I can have the bunbun react to the situation because it has some thought power behind what it's doing. In theory, I could decrypt hashes and modify my attack vector in front of the machine without having to make a second pass at the target.
Besides, go try and run a webserver on a Ducky. Let me know how that works out for you. One runs Linux, the other is a microcontroller.