I agree it is generally pretty stupid and difficult to watch, however there are a few situations where it makes some sort of sense.

1. It is sometimes used as a way to get around certain copyright issues (probably not legally, but it sometimes confounds the horribly stupid copyright bots that major platforms use with lazy impunity)
2. Sometimes the person “reacting” is themselves entertaining to watch. These reaction videos are generally for pre-existing fans of that person, not for fans of the video being watched.
3. Sometimes the person reacting is adding genuine value, for example they might be a legitimate expert (or at least a self-believed one) on the topic the video is discussing. This is in some sense sort of a video review, or a fact-check.
4. It is pure algorithm-bait, not really intended for any actual human to deliberately enjoy, but it tricks the algorithm into showing it to people who are too low-effort to look up the real video and just watch it because it’s in their “feed”.

Usually it’s some combination of the above.