Why should you be concerned about visitors coming back to your Web site? Because returning visitors are eight times more likely to make an online purchase. This data was not taken from wineries, but it seems pretty clear that getting a visitor to return (or otherwise interact with your winery) increases the chances that they will buy some wine.
This data speaks to the importance of creating a Web site that is visitor effective (a site where visitors accomplish their goal in coming to the site, and leave with a positive impression). Both factors influence whether a visitor will return to your site.
Unfortunately, most wineries don't have a good handle on how many of their Web site visitors are repeat visitors. Ideally, your Web site would treat repeat visitors differently that a first-time visitor, just as you would in the physical world. After all, a repeat visit is expressing interest and trust in your winery.