Combined with land tax, which thins the deck a lot to begin with, it's used for mana fixing (say with land tax I draw a mountain, a plains and a forest; I play a plains, and I don't have the mana of the specific type I need to play this, I play manamorphose and now I have the mana available on that turn, instead of waiting an additional turn to play that land), and also added deck thinning.
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Sorry for derping. Just casual. :).