The well made, expert reaction videos are fun. Like Legal Eagle reacting to videos to judge their legal accuracy. He also made a meta reaction video where he explained why some reaction channels were breaking the law and how to do it properly. Insider has a series of videos where they get an expert on a subject to react to movie clips (like a former bank robber reacting to heist movies or a marine biologist reacting to shark movies).
But I don't get the low quality stuff, where someone isn't an expert, doesn't provide meaningful commentary, etc. Some "reaction" videos are basically just stealing content and the only thing that seems interesting about them is the original content. The "reactor" adds nothing and just stole views from the original.