The answer is simple - because they work! 🙂 In my 20+ years in marketing, there was never a better tool for putting your offer in front of a potential customer. To understand why they are so wickedly effective, let's take a bird's-eye view of how the Facebook campaign works.

Everything starts from The Offer. We call it "lead magnet." You need one and it need to be sexy enough in order to be click-worthy. Free stuff, cheat sheets, checklist - short and sweet to attract attention.
You are giving it away in exchange for a basic info like name and an email address. That does few things - weeds out freeloaders, build your potential customer list and allow Facebook to start looking at the people visiting your page/offer.

And that's where the magic happens. Facebook is an incredible, sophisticated spying machine, supported by the most advanced personal database ever know to human race. Within few hundred visitors (a recommended number is 1,000), Facebook have a pretty good idea about your customer interests, behaviors, and activities.
From there you can get out off the way and let FB "hunt" your potential clients. All you need to do is to create new offers and tell FB to find customers with similar behavior to already existing visitors/clients/purchasers.

The more info you can feed into FB machine - the better output you will get. This process is the same no matter what kind of business you are in. And it works like magic.