Set Design needed the mechanic to help with "mana fixing," or getting you the colors of mana you don't have
You can buy gems in Arenas store for actual cash (or cash alternatives), earn them through Draft rewards, or occasionally through events
Athreos, God of Passage Ephara, God of the Polis Erebos, God of the Dead Heliod, God of the Sun Iroas, God of Victory Karametra, God of Harvests Keranos, God of Storms Kruphix, God of Horizons Mogis, God of Slaughter Nylea, God of the Hunt Pharika, God of Affliction Phenax, God of Deception Purphoros, God of the Forge Thassa, God of the Sea Xenagos, God of Revels
Stoke provides aggressive decks with other direct damage spells like Shock and Lightning Strike just enough burn damage for it to be a much more reliable win condition
Does the Legend Rule Apply to Tokens