Anti-fraud is somewhat different than traditional web application security. With anti-fraud, it's about recognizing behaviors and patterns that are likely to result in a fraudulent transaction (stolen credit card, mismatched information, sketchy browser, script/bot-like behavior etc).

In large scale ecommerce, fraudulent transactions in the 5 figures is so common it's not even notable. So using JS to reduce this even by half saves the company literally millions of dollars.