This is a very powerful feature that enables you to control what content do visitors coming from specific websites see. For example, you can choose to display one version of your website to people coming from Facebook and another version to people coming from twitter.
You can also go deeper and mix features. For example:
Displaying a different version to AMERICANS coming from Facebook.
Displaying another one to GERMANS coming from Facebook.
Displaying another version to AMERICANS coming from Twitter
Displaying another version to CANADIANS coming from Google that are already customers/followers/subscribers.
The possibilities to customize and group your audiences are endless. All this with the purpose of increasing your conversions and engagement.