Why do natural and geographical monopolies arise, and why are they often good for the economy and for consumers

They arise because goods can sometimes be found only in certain parts of the world. A famous example of this was historically silk which was only found in China and everyone had to pay what the Chinese manufacturers said. It was good for Chinese economy because their merchants and silk manufacturers were rich, and in return the consumers were satisfied because they had the original high quality silk coming straight from China which ensured quality.