If you are planning to distribute your collection among your heirs, or any other group you so designate, it is useful to have a listing of all of the items within the collection and the values of those items. For distribution planning the appraisal is not being submitted to the IRS and is therefore not held to as high of a standard as one that is to be scrutinized by the government.
For that reason it is not as relevant which values are being concluded, as long as the type of value is consistent throughout. It is really more of a tool for equitable distribution than anything else. However it will allow you to evenly divide the collection, or to divide it in whichever way you so choose, with the full understanding of how each item relates to the others in terms of worth.