Statistically, the difference between four stars and five stars in your aggregate Yelp score equates to a difference of 5-9% in revenue. However, for some local businesses, it can be far greater than that: a poor review from a single elite reviewer can sometimes sink an entire business.
People often go on Yelp looking for reviews on local businesses. It has become such a popular website that it is basically a search engine all of its own. If you have a good profile on Yelp, you will get more business through Yelp.
However, remember that Yelp does not exist in a vacuum. Poor reviews on Yelp often reflect real problems that need to be addressed within your business, and a poor score (or no score) on Yelp can have a negative effect on your ranking in Google and Bing.