Spells you cast cost 1 less for each card your opponents have drawn this turn
(Additionally, some of these effects are often built into tribal rewards on creatures for the tribe, such as Heron's Grace Champion being a combat trick in addition to also being a creature.) Additionally, there are cards that want your deck to have a higher overall count of a creature type to make sure they work more often
If you happen to be building a strict theme deck, you will probably want to dip into almost all of these cards, as well as Might of Oaks (the Urza's Legacy, version), Monstrous Growth (the Portal Second Age, version), and the amazing Squirrel cards before Odyssey
Who can say why, maybe its because our toddlers came in and gifted us with an illness, who can say
Dracula by Bram Stoker