Processors do their own fraud as well. But, as they're based on heuristics, the more context you can give, the better. Depending on how exactly they do fraud, they may not see a specific kind of fraud coming through one customer, but may be better at seeing fraud spread out amongst customers.
You know your customers and business better than anyone, even your own upstream.
Preventing fraud is the most difficult part of payment processing; are they outsourcing it to the merchants?