Why You Need Snow

In Vintage, Snow-Covered Island and Island may as well be the same card. Arcum’s Astrolabe sees no play. No one is going to Icequake you.

But they are not the same. It matters in Doomsday. Doomsday plays nothing that cares about the Snow-Covered supertype. That said, there is one important distinction: Demonic Consultation.

When siding in an extra Island, knowing one is Snow-Covered and one is not allows you to name either. This matters and can lead to game-winning plays.

Imagine this:

Your opponent has Archon of Emeria in play. You have Demonic Consultation, Thassa’s Oracle, and Preordain in hand. Three Underground Seas in play. Ah, you could win, if only that darned Archon wasn’t there. One spell a turn.
You cast Preordain and see Island and Dark Ritual. You put them in that order on the bottom of your library. During your opponent’s next end step you cast Demonic Consultation and name Island. You now have one card left in your deck, Dark Ritual, which you draw during your turn. You play Oracle and win.

But Mono-White Initiative is a mana-denial deck. Adding another Island in games two and three is not a bad idea. If your sideboard contains another Island, this would not work. (Unless, of course, the second Island was in play or in your hand.) These slim margins are what makes Vintage.