Dumping three or four lands while your hand is full of them can catapult you ahead of your opponents in terms of resources very early on, giving you a distinct advantage in the early game and potentially making you a huge target at the start of the game
Since Sagas auto sacrifice once they hit the final chapter, generally only sticking around for three or four turns, you need some sort of engine to keep the Sagas on the battlefield
The player who draws Wrath of God in a game of Sealed Deck has the ability to play the game in such a way that the Wrath of God hurts him much less than the opponent
Getting another card that doubles them on a card that can easily slot into any deck featuring blue only adds to the archetype
This means if you cast an instant or sorcery spell, then Baral and Kari Zev comes under your control, its triggered ability wont trigger that turn
Legacy is for the players who want a stopgap between the most powerful cards of Vintage and the most recent cards