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First the obvious - greater interest potential -- a well written email is well received by someone with a current interest.
The not so obvious - the owner of the list are generally known by the opt-in audience - thus quality control must be maintained by the holder or risk alienating there niche product.
The owner of the list is more knowledgeable about their opt-inners. They have a greater appreciation of the likes & dislikes, where a general mailing list owner really doesn't care (a jake-of-all-trades, master of none sort-of-speak).
Do industry related searchers - I'm sure you will find lots.