One of our engineers today overheard the following on his commute (said by one business owner to another):
"Yelp keeps you on your game"
Any customer that walks into a local business may be a Yelper, as a result owners now have an even stronger incentive to perform their absolute best for every single customer. When this happens owners reap the benefits of the positive word-of-mouth that is then reflected on Yelp and the marginal customer gets a better experience. I call that win-win!