I know what they used to be for. A decade ago, there really was no way to tell if a movie, a book or a play was worth your time before you paid up. A professional review could be a valuable signal, a way to save people time and money.
Along the way, professional reviewers also decided that they could alter the culture by speaking up. Since creators of culture are often sensitive to what the critics have to say, establishing critical baselines (particularly when you are a powerful arbiter of what sells and what doesn't) became a real function of the critic.
Today, of course, there's no shortage of cultural feedback. If I want to know what people thought of a bit of culture, it's only a click away. In fact, for the consumer who doesn't want to know (spoiler alert) it's almost impossible to avoid.
With that much feedback to choose from, what purpose do the anonymous book reviews in Publishers Weekly or Kirkus Review serve? Or the long movie reviews in the Times or the short ones in Variety? Or the restaurant reviews in the local paper?
They might be saying, "I have a track record, and if you agree with my past picks, you'll agree with this," which works fine if it's always the same reviewer and we know them by name.
They might be saying, "our publication has a good track record in picking what's going to be popular, so if you're a theater owner or a bookstore, pay heed," except they don't have a good track record, they have a terrible one.
Or they might be saying, "attention actors and directors and writers–we don't like it when you make books and movies that we don't like, and we're going to pillory your work until you stop." Assigning someone who doesn't like an author's work to review the author's next book seems cruel to all involved.
[And sometimes, they're just fun.]
All a long way of saying that if you make something that people are likely to criticize, pay careful attention to which critics you listen to. They probably don't view the world the way you do, and worse, the way your fans do.
Reading criticism just to ruin your day is a waste of your talent.