For people who use Facebook as a way to, you know, communicate with friends, this feature might sound kind of ridiculous. Why pay extra to share that funny YouTube video that someone else found with even more people? The social network, however, has some suggestions:
If your friend is running a marathon for charity and has posted that information publicly, you can help that friend by promoting their post to all of your friends. Or if your friend is renting their apartment out and she tells her friends on Facebook, you can share the post with the people you and your friend have in common so that it shows up higher in news feed and more people notice it.
In other words, it's for the more utilitarian Facebookers, and apparently enough of those exist for Facebook to make money off them.