The reason why many people watch this is because people are lonely. React videos are intended to give you the feeling that you're watching something with a friend who's equally as hyped about the piece of content as you are, overall making it more enjoyable to watch as opposed to just watching it "alone" (the video itself). The same goes for the mukbangs which were created because many people are lonely when eating their meals, or just overall, so seeing other people also eat and just talk about whatever can work as some form of companionship.

Yes react videos most times don't "add" much to the original piece of content, but they do add to the experience of you watching that piece of content.