When you increase your deck size, you lower your chances of drawing your best cards
With black, you'll likely have some removal that will be able to deal with any threats during combat or reanimation to bring him back from the graveyard
Because it provides both that early defense and later-game shenanigans (and combines well with stuff from Fungal Bloom to Doubling Season), this is a potent defender for your next deck
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This only works with Swamps, but if you have a card like Urborg, Tomb of Yawgmoth, which turns all your lands (and your opponents lands) into Swamps, you have a ton of mana at your fingertips
Veiled Sentry I find it funny that Veiled Sentry can prevent opponents from casting their biggest spells and instead have them cast a cheaper one first