Affinity is a fast artifact-aggro deck full of cheap evasive threats like Glint Hawk and Kor Skyfisher, so lifegain to survive the early clock matters more than card selection here. Weather the Storm's storm copies let us gain significant life on the turn we're going off, which can completely nullify the small amounts of damage they've been chipping in. First Day of Class is replaceable since its counter/haste synergy and Learn value matter less against a deck with few creatures to buff and no graveyard interaction for us to worry about. We generally just want to race to combo off before their artifact creatures overwhelm us.