Review sites help people to overcome uncertainty about the standards of service, food and cleanliness a restaurant provides.
This is especially important for relatively unknown establishments just starting up.
Review sites have grown in size & number considerably over the past ten years. The number of people using review sites to make dining decisions has been a driving factor in their success.
According to a Harvard Business School report*
“a 13% drop in sales after the first bad review”
“a one-star increase leads to a 5-9 percent increase in revenue for independent restaurants”
*source: *The Case for Yelp”, Michael Luca http://www.hbs.edu/research/pdf/12-016.pdf