Here's an example of a boosted post from Bustle, who promoted one of its articles on Facebook. Paying to "boost" a post you already posted organically to your Facebook Business Page can greatly benefit content that has mass appeal -- versus a post that targets a specific segment of your audience. Bustle's choice of boosted post here falls into that first category.
You also have the option to select a Custom Audience -- this allows you to target people on Facebook who are in your company's contact database, visited a page on your website that has a tracking pixel, or use your app or game. To learn more about how to set up a Custom Audience on Facebook, check out these instructions. (And for more on the specifics of these criteria, visit this Facebook targeting resource.) How to prepare and market using Facebook Ads