While it may be useless, I can think of a situation in which you might choose to buy the "useless" card instead of another card.
You have Village, Black Market, Royal Seal, Province, Province. You play the Village and draw an Upgrade. Now you play the Black Market, and find two cards worth 3 and the Treasure Map worth 4. You take the Treasure Map, use Royal Seal to put it on top of your deck, and then Upgrade to draw it and make it into a Duchy.