Quite simply, young Wizard,Treasure Cards are single-use spell cards. Once you use them, they're gone. If a Treasure Card fizzles on cast, it is placed back into your deck.
Since treasure cards only have one use, I do not know when best to use them.
I don't want to waste a card that *might* come in handy later.
Something that only works once seems so precious that I do not want to use them for regular battles, or even PVP. And boss battles are often challenging to solo, but not impossible to win, so I don't use a treasure card there either.
I suppose after I collect a dozen of the same card, it won't seem like a waste to use one, but for now, I only have 1 or 2 of any card.
Some people NEVER use Treasure Cards, but sell them for gold at the Bazaar. Some people collect Treasure Cards, trying to get one of each various kind available. Everyone has their own theory about them, but if you never use them because you think you might need to use them, you're kind of stuck in a vicious circle. Also, carrying lots of Treasure Cards has been suggested as a cause of the game taking longer to load as you change areas, so you may want to start using them just to improve game performance.
