You can, in fact, reach certain users who are not yet engaged with your Facebook business page. Organically because of Facebook's EdgeRank algorithm, a page's posts will only reach about 16% of your fans' news feeds. But Promoted Posts are not only engineered to show up in a larger percentage of your current fans' news feeds. In fact, when one of your fans interacts with (i.e. clicked/commented on, shared, liked, etc.) your Promoted Post, it will also show up in the in the feeds of their friends, even if those friends aren't directly fans of your page. The post that would show up in these second-degree connections' news feeds would show the interaction between the fan and your business page.