The black bars fill where you duplicate the video content onto a lower layer and magnify and blur it is one of those very stupid effects that are used because to a toddler’s mind they seem like a great idea. But it provides zero benefit. It reduces compression efficiency by leeching off bitrate and generates a constant distraction for the watcher’s attention through pointless movements over the majority of the picture, which is especially bad because human vision is more sensitive to movements towards the periphery and because the zoom amplifies the actual movement energy of the source video. It also blurs the perceptual boundaries, making it more difficult to clearly and easily identify where the actual informational video content ends. This is even worse on videos that are intended to be fully informational and not artistically playful, like for example this one:
In short, it is added effort to make a video a PITA. (In German this is called Verschlimmbesserung. Could be translated as worseprovement.)
Here is a good video demonstrating how easily attention can be diverted: https://www.youtube.com/watch?v=Ahg6qcgoay4
(It is a somewhat different case, but it is about moving elements drawing attention.)
UPDATE: I recently encountered a typical result of when people follow some trend thoughtlessly; a 16:9 video with mixed media content in which all 4:3 (so not even 9:16) aspected concent had black bars fill, but cinemascope content did not have it for the black bars on the top and bottom. Suddenly black bars are totally fine. Why? Because everybody leaves them black.
This is just one example for many common inconsistent habits that show how people can be programmed into behaviors if they don’t question them.