The main difference between an appraisal based on rebuild value and market value lies in the purpose of the appraisal. An appraisal based on rebuild value estimates the cost to rebuild the property in the event of a total loss and is primarily important for insurance purposes. It ensures that the property is not underinsured or overinsured. An appraisal based on market value, on the other hand, determines what the property is worth in the current real estate market, crucial for buying or selling the property, obtaining financing, or for valuing the property in financial records.