So that they can attempt to get the value of the contract, do you know why? Banks are in the business of lending money, no loans, no income.
As a mortgage officer, the only time I am really frustrated by an appraiser is if they come in < 1% low. They already have been given a 10% tolerance on erring by the real estate board, just say it is the value and move on.
That being said, other than VA MORTGAGE appraisals which are notoriously bad, if they are low it is because the data on the MLS only supports the lower value, because the data on most MLS does not include all sales, it is dirty.