Reaction videos seem like a pretty new phenomenon. Someone films themselves watching something and offering close to zero insight or adding anything at all. Often the videos are monetised, and more commonly what they're reacting to is from YouTube and is interrupted with advertising.
There is an exception. Watching younger people listening to music that I have known for decades for the first time is fascinating. Seeing how they react and really appreciating what it is, though possibly embellishing their thoughts for the sake of the video.