There could be no. of the reasons why one create a Fake account on Facebook.
Some of them do it too hard others while others do it for fun and not particularly annoying anyone in that process.
Spamming and promotion of their stuff.
1. FB blocks the accounts if someone tries to penetrate their securities so Hackers usually create FB fake accounts for hacking as no one wants to risk their original account. (One of my friends has lost his account while doing some foolish stuff on Facebook)
2. I have two FB account, one is real and other is for testing purposes. (nothing blackhat)
3. Then there are people who had 4-5 FB accounts just to get more promotional offers from FB. (For example, Freecharge give 50 rs free recharge when you log in with your FB account)
4. Some advertised to make a fake account for displaying ads on Facebook. (not sure why they do that)
5. Psycho lover sending spam message and comments to their crush and stalking them on FB. (very bad)
But there are couple of ways to find out if the profile is fake or not. The best of all is reverse Google search the profile images. More can be find here at How to Identify Fake People on Facebook