would you just say "ok, that guy knows more about my home's value than me, a willing seller, and that guy, a willing buyer"? This is the definition of market value.

When the government "fixed" the lending industry because of the housing crash, one of the things they did was require appraisers to be assigned randomly to appraise properties. They felt that there was collusion between appraisers and lenders to get improper values for cash. (there was, but not systemically). So, you'd get these random clowns from areas far away to come in and appraise a home in community they knew nothing about. I had an experience with a $650k home in the SF bay area where an appraiser was assigned out of Modesto (which is NOT the bay area). He came out and said to my face that there was no way this home was worth $650k because where he's from you can get better ones for $250k! He lowballed the appraisal at something like $600k because I GAVE HIM COMPS he couldn't deny, but he was still ticked about it. In this case my buyer was the one that was affected, he wanted the home for his family, knew it was worth $650k all day long and was more than happy to pay that amount, but the bank denied his loan based on clown's value. Luckily, the buyer was able to bring $50k more in to the closing to make the deal. The seller would've just walked away and sold it to the next guy and hoped that the next guy's appraiser wasn't an idiot.

No, appraisals are an art, not a science. And in many cases they're just flat wrong.