What this means is that, should the Troll take lethal damage from something (say from combat, or your opponent casting Pyroclasm), the Troll will stay alive as long as you pump two mana into him
each one allows you to play with a more diverse mana base, and that, in turn, allows for more consistency, a wider breadth of decks on the whole and a more exciting game
Card Type Distribution The Saga Tribal Card Type Distribution shows the dominant enchantment-heavy theme with 36 enchantments (if we count the 14 enchantment creatures)
With flashback, cards like Increasing Ambition and Fires of Undeath are one(ish) more mana than you would pay for the effect without flashback
They may occasionally pop up in competitive EDH decks, the most notable being whenever Rick, Steadfast Leader shows up once in a blue moon in a Legacy Humans decklist