Some online reviews may raise concerns or suggest improvements related to your business. This can be a good opportunity to resolve a customer's problem and/or improve business practices. After all, many of the customers who raise concerns in online reviews would still complain about your business to other people, even if they didn't post a review.
At least if the complaint is published where you can see it, you have the opportunity to address your customer's concern and respond publicly, if you feel this is necessary. Many customers also have good suggestions about better ways to do things, or new products that you could offer, so review sites can be a free source of great business ideas.