If we can sneak it onto the battlefield on Turn 2, every other spell in our deck comes with the kicker of milling our opponent for two cards
Megaton's Fate has some versatility, but its first mode is basically a Shatter at sorcery speed, and its second mode isn't necessarily going to sweep the board (and those four milled cards aren't doing much either)
Anytime a creature you control enters, you get to put a +1/+1 counter on each creature you control
With 36 lands, you have a 90.6% chance to draw 2 (or more) lands in your first 9 cards
And although its too powerful for Standard, stepping aside for Cancel and variants that might fire for the 2 blue under the right conditions, like Ice Out, it remains important across formats
Effects that sacrifice creatures such as Goblin Bombardment take advantage of this to turn a normal board into a huge army of elementals