An appraisal is different from an assessment. An assessment is done by the taxing body, usually your city, and is used to assess property taxes. An appraisal on the other hand can be ordered for many reasons. For instance, a bank will order an appraisal of the house before they refinance a loan to make sure the value is there. An appraisal will tell a lender the replacement value of a home or whether the home is a good business decision for the bank. It can be ordered to help dispute a property tax increase or to prove a replacement value of the property to an insurance company. And of course the bank will appraise the house prior to making any loan to a new buyer.
An appraisal tells you what the lender thinks your house is worth but it doesn’t tell you what a buyer will pay. A buyer will base their opinions of what the home is worth on their agent’s comparative market analysis, not an appraisal’s report. An appraisal can give you a ballpark figure upon which you can base your asking price but the price you receive may vary.