The other card will be a rare or mythic rare, which could be a stained-glass legend, an extended-art card, or a borderless planeswalker
an opposing artifact deck could win the game by attacking for lethal, or your artifacts could be destroyed by a common board wipe aimed at creatures like Damnation
A large burst of mana plus three fresh cards often leads straight into a combo turn or a massive swing
Additionally, if your deck has a lot of card draw or even just manipulation (your Commander Scrys or draws, for example, or you're Mono-Blue with a ton of cantrips and Ponder-style effects), you can adjust your number
These decks have terrible mana that involves lands coming into play tapped
Yorion, Sky Nomad is still legal in the Pioneer format and Keruga, the Microsage and Obosh, the Preypiercer see some Modern and Pioneer play