Dig UpDig Up can be cast early to find a basic land, but if you get it back as your one mana card when you cast Seasons PastSeasons Past, you can then cast it for its cleave cost to search your library for Seasons PastSeasons Past (which shuffles back into your deck), and you can do that all over again
The Great Henge isnt at its best here, but as The Party Tree box topper, it still is likely to cost 4 mana or so in the mid-game and provide card advantage, mana ramp and consistent life gain
But then in Fifth Dawn we went and made Vedalken Orrery
It seems to me that he would prefer to bide his time, sculpting your graveyard for the perfect moment to unleash the might of your deck on your opponents
He doesn't offer the same kind of disruptive potency
Its a removal spell for decks that lack them, especially 100% colorless decks