Why do I need a professional appraisal?

Why do I need a professional appraisal?  

Anytime the value of your property is being used to make a significant financial decision, an appraisal helps. If you're selling, an appraisal helps you set the most appropriate value. If you're buying, it makes sure you don't overpay. If you're engaged in an estate settlement or divorce, it ensures that property is divided fairly.