As a colorless card, these tend to see play in midrange decks that need some card advantage or that interact with artifacts in some way, like blue-based artifact decks, or just colorless decks that jam as many artifacts as they can
Some rebacked cards are made when the back of the card was considered marked or unplayable, and someone put the front on a new back to allow the card to be "playable" again
Then since Ur-Dragon is now Skithiryx pumped up by +6/+0 when it was Moltensteel Dragon, you can swing with Skithiryx as a 10/6 to deal 10 infect to your opponent, which causes them to lose the game
This foil Karn was only given out to Top 32 players at the Pro Tour
In terms of overall numbers, the Elvish Rings are the rarest, limited at 300
The rub here is you have to pay that life, so you'd need a way to keep the storm going to protect yourself