Unless players have cards that can instantly banish a character from play, Steel cards will be tough to get rid of, especially with some having the Bodyguard feature that draws challenges to them instead of more vulnerable characters
But most importantly, instant speed might allow this to double up as a combat trick, and the extra +1/+1 counter adds to that
If all you want is to find one card you can find it but the Vault offers something more by letting you preview what cards would come along with it, allowing players to plot out the next several turns of the game and see if the sequence being offered would get it done
This wall basically fogs a creature
Second, being more expensive can be an advantage to a threshold card because that gives you time to get the requisite cards into your graveyard assuming that you're being rewarded for all your extra effort
She wears a ring on each finger, each of them filled with poison