Maybe it's because your opponent has forced you to with their own spell
Rather like how you can try to bait out a board wipe by applying enough pressure that your opponent has to cast it without playing all your creatures, a bit of self-mill can prompt your opponent to crack these before they want to
The catch is that exile replaces the graveyard afterward, so timing matters
The discover value equals that creature's toughness, letting you exile cards from your library until you find something with that mana value or less, then cast it for free or put it in your hand
Then in your combat you can flip her into an Anthem to double pump your legendary team, but only if you control two more
Notable Reprints Avenger of Zendikar, Taurean Mauler, Tendershoot Dryad, Raise the Palisade, Kindred Summons, Coat of Arms, Maskwood Nexus, Sol Ring, and Bitterblossom get Secret Lair art themed around a massive party