It allows cards to attack and deal damage to the opponent without blocking
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
//// Prowl, Pursuit Vehicle Color indicator: white Legendary Artifact Vehicle 2/3 Living metal (As long as it's your turn, this Vehicle is also a creature.) Whenever another creature or Vehicle enters the battlefield under your control, put a +1/+1 counter on Prowl
Needless to say, preventing your opponent from special summoning from the extra deck for the price of 1 card makes me wonder if it should be banned
Plague Rats, most obviously, is non-functional in a context of 60 card minimums and 4 per card maximums, but is hilarious and brutal in a 40 card minimum environment where an opponent may have a dozen or so Rats
The player can detach a material from it to destroy a monster on the field, then make the opponent take damage equal to its attack