Validation rules catch problems at the door, such as missing required fields, dates in the wrong format, totals that do not reconcile, and values outside expected ranges. Rejected or flagged records go to a person for correction instead of silently corrupting reports.
One principle matters especially in finance. Missing data should stay missing, not be treated as zero, and every calculation that depends on it should say so. A forecast that quietly assumes zero for an unknown payroll figure looks precise and is simply wrong.