Why are error coins valuable?

Hi all,

I've inherited a decent coin collection, so I'm new to this world. Sorry if this is a n00b question, but why are error coins valuable in the collecting world? Is it an issue of rarity?

Deeply appreciate any insight!

 

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Rarity, wow factor or both....

 

I think it's mostly rarity. I mean, all error coins are one of a kind pieces. And of course the greater the error the greater the value.