Vona, Butcher of Magan sports the knight subtype and is in black and white, the two best colors for a Commander deck based on knights, so it became a way to enable these decks, even if the commander itself didnt see all that much play
I believe it was Richard Garfield's way of creating juiced, above-the-line power level cards with an ongoing drawback to enable a balanced card
Don't forget that you can mill yourself too, letting you use Brain Freeze to fuel graveyard-based strategies or self-mill win conditions
In fact the printing of Unholy Heat (and to an extent Solitude) has mostly pushed out cards like Tarmogoyf out of the format, as the removal in the format is now so efficient that creatures either need to be one-mana, so you trade at an even level (Dragons Rage Channeler and Ragavan, Nimble Pilferer), too large to be killed by Unholy Heat (Deaths Shadow and Murktide Regent) or generate immediate value upon being played (Solitude, Fury and Omnath, Locus of Creation)
For five mana, Mulldrifter is a 2/2 with flying that will draw you two cards when it enters the battlefield
Let it die and bring it back again in a loop
