Sale or Purchase Consulting
Since appraisers do operate within a context of valuation services, and have the most insight possible into all of the relevant markets for a given item from retail to auction and every layer of those different markets, it is to the advantage of someone considering either selling items or building a collection to make use of an appraisers services.
Of course a dealer or gallery might know what a work would sell for if they were hanging it on their wall, and they would certainly know how much they would be willing spend on an item for their own inventory, however they might not be the most reliable when it comes to advising someone in a sale or a purchase.
Having an impartial opinion of value is a very worthwhile advantage when deciding if something is worth buying or where best to sell a given item. An appraiser can and will provide you with the most accurate value of an item, and the various values any item would have in whatever relevant market it appears.