Before the Bracket System, Jason Alt created the 75% rule, which was a way to build decks around fun and power
This deck would cast Squandered Resources, sacrifice all lands to Natural Balance, then cast Cadaverous Bloom, and then pitch cards to generate mana and draw more cards
By itself, it only provides benefits when it enters the graveyard from the battlefield
It also makes your dinosaurs incredibly powerful chump blockers, since they can deal damage back to the creature that attacked it, or directly to other players
Starting them out as uncommons builds hype to possibly see them later on
Athreos is cheaper than most of the other options out there