It can Rampant Growth for another mana with Fertile Footsteps and then later arrives to the party with a 7-drop X/X equal to your land count, which count be quite a lot
Most notably if you play Icetill with ~15 lands in your yard, they show up in the middle of your screen, pushing your hand off screen to the left, so you can no longer play cards from your hand
Finally, their ultimate allows you to gain seven life, draw seven cards and then put seven permanents into play for free
it can be tempting to blow a lot of removal and countermagic on the early turns, but you need to save them for really impactful threats that will force their way through your value engines
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My current inclination is that Mangara, the Diplomat is great in the 99 of any Mono White deck and some Boros decks too as generic card draw