They are for spamming facebook. The big problem with that is, facebook likes to sue. Also, be careful because some of these guys selling accounts with friends, are selling stolen accounts they phished, and that wont bring the type of attention your looking for.
Or, they could be filled with bots as friends.. which is also useless. Sometimes I wonder just how much of facebooks user base is bots.. 5%..10%..50%?
Spamming facebook isnt what it used to be. Some people were making millions of dollars per month a couple years ago, but none of those ways work anymore.. your a little late. Now any links posted or injected to facebook will be blocked before you can count the spam recipients on one hand.