While giving out free Wi-Fi, you can Promote your own Facebook Page, get new "Check-Ins" and social recommendations from your customers. This increases your business' visibility, customer engagement and sales.
New "Check-Ins" means more engaged users. According to Facebook the average user has 130 friends who can also see your business once their friend checks in. That means, depending on the number of fans you have, multiplied by the number of friends they have, your content could achieve wide viral syndication.