The rarity of the Top 10 cards means only serious collectors are likely to have them in their collection
It's needed in case a player's main card is put into a library, hand, graveyard, or exile
7/132 Giovanni's Nidoking(Holofoil)
While non-NM copies of this card only sell for a few pennies, NM copies can reach as much as 39 cents
Wheel of Fortune, Windfall, and Brainstorm are but a few of the common wheels (cards that cause everyone to discard their hand and redraw a full one) and low mana value card draw engines that get this swarm humming, and if you run out of quick card draw options, 4 mana will let you loot an extra card in your deck to try and find the next one
Once your Vault hits 100%, you can open it to get more wildcards