Posting Yelp reviews encourages others to give their feedback on Yelp.

Posting positive reviews also encourages customers to leave more positive feedback than if they had not seen any reviews.

Not only will this encourage more positive reviews, but it will even affect the experience of all your customers. It's purely a psycological trick, but if your customers know that others had great service at your restaurant, then they will assume that their service will be great as well and be willing to cut you more slack than usual.