Iconic art, nostalgia, and historical importance can make collectors want a card even when they are not using it in a deck
For instance, in Mono Red, out of the 70USD total price, 20 is just a playset of Great Furnace
Its 5 mana and requires you to get in with attacks, but once you start getting the big swings in, youll be cheating mana costs on your opponents most powerful artifacts and creatures
You can untap it by putting a -1/-1 counter on it, so pair it with something that offsets the counters or turns it into a non-creature, and youve got infinite green mana
It is a bit situational, requiring you to have a completely open opponent, but each time you deal damage to a player with a creature enchanted with Breath of Fury, you get an extra combat step at the cost of sacrificing the enchanted creature, reattaching the enchantment to another creature
You remain the controller of the battle, even if you don't defend it