What is difference between government appraisal and landowner appraisal?

The government - in the form of the local tax appraiser - comes up with a value (i.e. appraises) property so that the government knows how much tax to assess that property. Alternatively a landowner can hire a private appraiser to give the landowner an idea of how much the property might be worth or sell for, a value which could be useful in determining how much insurance to have, or whether to refinance, or estate planning reasons, etc.