Adding it to your website will encourage readers to share your content with their individual networks on Facebook. Doing so will expose your content to potential fans and customers outside of your direct network and expand your reach.
The "Like Button" is available for use on websites outside Facebook: "When the user clicks the Like button on a site, a story appears in the user's friends' News Feed with a link back to the website". At the same time when any visitor, including non-Facebook members and logged out users, visit a site with the Like Button, their presence on the site is recorded by Facebook.
Incorporating the Facebook Like button to your website can serve you well in terms of increasing traffic. Each time visitors click the button, information about your site is shared with their friends via the News Feed, which can result in more traffic and generate more leads and referrals.