[swearfu]

Charge backs. For a long time those industries were dominated by affiliate programs, and so affiliates would hire firms in other countries to use stolen credit cards to inflate memberships, then charge back after the affiliate got their payout. By using shared networks of the finger prints we were able to identify computers already used for fraud on other sites or our own sites and refuse them the chance to even sign up.

EDIT: Sorry, to be clear, the people using the stolen credit cards didn't charge back, the actual owners of the cards would. But this affected the industries two-fold, first in constantly having to battle with credit card and billing companies that didn't want to provide services for an industry with such a high charge back rate, but it also hurt us in paying out to affiliates who cost us money instead of bringing it in.