The news article Five things to know about home appraisals: Getting one before a home hits the market can save time and money [MW]  is a pretty good summary of the advantages of getting an appraisal done before you list your property, if you can get beyond the fact this appears to be a planted public relations piece by AI and ASA than investigative journalism. But thats ok because its actually pretty accurate. In fact, both organizations provide some good insight into the appraisal process in this article.
One of the biggest changes in the market today is the fact that accurate pricing has become a lot more important. Because of the increased competition of having more houses on the market, gone are the days of throwing a high price on a property and see if there will be any bites. In fact, now such a property gets stigmatized for having an unrealistic seller may actually ultimately sell for less than if it was priced accurately.
The key advantage is that the appraiser is independence from the sale process and gets the same fee no matter how high or low the property is appraised.
However this is a moot point if the appraiser is not well-versed in the nuances of a particular market.
Real estate brokers are on the front lines and may have access to great information based on first-hand experience. However, no matter how good they are, they are connected to the ultimate sale and are perceived to be tainted, despite whether this is fair or unfair, accurate or inaccurate.
We see this everyday in the legal system. The courts generally do not accept a broker’s testimony as unbiased simply because their compensation is based on a commission.