In short, if you get the landfall trigger in your first main phase, your creatures wont untap for your actual combat phase once the extra combat steps finish
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Old One Eye Eleven power in tramplers (as well as giving it to the team) for six mana
That's also a time to do two-mana ramp, because if we get lucky we can play two spells on turn three, but it's also a reason for Cultivates (when they're available) because that guarantees a Land drop the next turn in addition to ramping
The card also acts as a win condition as well as card advantage, which is generally all you can ask for
The main reason for this is that you do not want to expose your creature to graveyard hate before you need to try to put a creature into the graveyard and reanimate it on the same turn