It seems to let you "Prepare" any one spell on your spell list and put it on a card. So a Wizard could prepare his usual spells, then pick another spell he has not prepared and put it on the card. Alternatively, a Warlock or Sorcerer could pick any spell from their list even if it is not one of their Spells Known, which is pretty huge.
The free Quicken is gravy.
The free Quicken is gravy.