What renegade is referring to is news feed position of a post. Facebook has 2 sort settings, most recent and "top stories". No matter how many times you select most recent it will always revert back to "top stories" after a short while....and this is intentional.
Promoting a post makes it a "top story" that out ranks the more popular posts appearing in someone's news feed. When people have a lot of friends, and therefore a lot of news feed posts to read through, and maybe play games with a lot of game posts in their news feed, too, they are more likely to miss stories that are a lot farther down the page. Being a "top post" will put it near the top of the page, where they are more likely to see and notice it.
All it does is make sure your post isn't buried waaaaaaay down at the bottom when subscribers log in and view their news feed.
This is why I said what I said about commercial posts and number of subscribers dictating whether or not the $10 is a good deal.
It'd be nicer if they actually said how much people is certain to actually see the post.
It not based on impressions, like ads. There is no real way of knowing how many will see it. No way of knowing in advance how many of your friends will log in and visit their news feed page on any given day. And it's also possible that some of your friends might even be blocking your posts from appearing in their news feed. (I have a few relatives I have done this to, that post a lot of stuff that I find personally offensive) It's more comparable to a search engine ranking position on an endless page of results. You want to go natural and be a few miles down the page or pay $10 to be within the first 10 on the page? Even if you pay to be #10, there s no way of knowing in advance if people will close the page after viewing the top 3.