The short answer
Past flooding affects insurance availability, premiums, lender decisions, and resale potential.
Even if the flooding happened years ago and the property has been fine since, insurers and lenders often still factor it in. That history can mean higher premiums today and a smaller pool of buyers when you come to sell.
Why this matters for your purchase
PropM8's environmental section looks at flood history as well as current risk, because that history is what shapes insurance and lender appetite going forward.