Domain knowledge tells you which combinations of columns mean something, and a blind search over raw columns cannot guess that. In card payments, transaction amount alone is weak. Amount divided by that customer's usual amount is strong.
It also tells you what exists at decision time. Someone who runs the process knows the risk score is written back two days later. That single fact prevents a leaked feature from ever being built.
It flags proxies too. Postcode often stands in for income or ethnicity, which matters for fairness and for stability when a neighborhood changes.
Without it, you brute-force thousands of combinations and hope a selection method sorts them out. That is slower, noisier, and produces features nobody can explain to the team relying on the prediction.
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