Have you ever used a local service online rating app on iOS or Android like Yelp, Google Local or Foursquare and did an action based on these reviews? Some people might be going to a new restaurant because of all of the great reviews. Others may avoid a hotel because of the low online rating. If you are really ambitious and trusting, you might even go to a specific doctor or get a lawyer based on reviews from strangers.
The problems that has risen is when you pay a company to write good reviews or negatively review competition unfairly.
In California, this is now illegal.