A secret for Legendary Fish. If you read the 1.6 book, which makes it possible for any fish to yield roe inside a fishing treasure chest... Then you can find some, if you are lucky! Legendary fish cannot be used inside a Fish-Pond, so you cannot have them "produce" stuff; which means their roe is rare and is considered to be, something like a "trophy" of sorts!
You can also use that lure for making the caught fish of higher quality. Just imagine, an iridium quality Legendary fish!