You can’t craft with this Enchanted Weather Undermine Crest (616-629), so whats it used for?
You can. The Enchanted Weathered Undermine Crest is used for Rare (Blue) quality items.
Items that require a Spark will not take the Crest in question.
Provides a nice boost to ilvl of whatever slot you put it on, costs no Vstones and 15 weather crest less than upgrading it with the two lizards.