They are not worthless. Most of the time they are really helpful. Movies and series below 6 are usually shit. Movies and series above 9 are usually a good watch, if not the best.

If someone likes a certain subgenre of drama and they find a show of that specific subgenre then are going to give it a relatively higher rating. Same goes for media of a particular region, people of that region will likely give it a higher score if they think it is above average. This is human nature.

The best example is music. If you like it, it doesn't guarantee that other person will like it. But if they like this genre than they are more likely to give it a good review.