In a home sale, apart from the actual buyer and seller, other parties involved include the lender and the local government. The lender wants to be sure that the mortgage is the right value and has been given to a legitimate customer.
Local governments want to ensure that the price is right for the purposes of tax collection.
The lender is also interested in the deal as he will be filing for taxes based on the amount given.
In the unfortunate event that the transaction proves to have been a fraud, the lender will be counting losses.