It's not just convenience and speed. In hot markets, the appraised values can't keep up with market prices (since the comps are a half year old). If the buyer doesn't have resources well above the minimum to make up for the difference between the appraised value and the sales price, it's likely a loan cannot be approved. Before 2009, appraisers would just appraise at the sale price because the real estate agent that hired them wouldn't hire them again if they didn't.