Lost your password?
It's called afiliate marketing. Depending on the scheme, they will be paid for the click or in case someone is stupid enough to actually buy something off the referred site.
Of course, referrer spamming is a complete waste of time and resources if nobody ever sees those referrers because they're buried in a logfile. I don't see that much referrer spam any more, so maybe the spammers are slowly getting the clue.