loan officers and underwriters are not always on the same page. One takes your information the other verifies it. The loan officer who wants the loan to go thru, as that is how they are paid, orders the appraisal. The underwriter makes sure on your ability to pay the loan back and verifies all that you provided your loan officer.
Appraisal just states the value of the home in the market place, as stated below, if it does not meet the price you are paying they can offer you a lower loan amount, then you have two options, come up with more cash, or see if the seller will take a lower price.