Some players argue that perhaps Griselbrand didn't really need another reprint after being a Grand Prix promo. But as Griselbrand is a fan favorite, it's not really a surprise that Wizards of the Coast decided to feature him on the set's packaging and as one of the chase mythic rares of the set. They know that this powerful Demon will help sell the set.
As far as Modern is concerned, Griselbrand is part of only really one competitive Modern deck right now. That deck is Goryo's Vengeance, a deck that utilizes the deck's namesake card, Goryo's Vengeance, and Through the Breach to cheat out Borborygmos Enraged, Worldspine Wurm, and of course, Griselbrand.
In Legacy, Griselbrand is featured in multiple decks, including Sneak and Show and Black-Red Reanimator, as well as other decks. Even in Vintage, Griselbrand is a part of a powerful deck archetype called Bomberman Oath, which utilizes the powerful Enchantment, Oath of Druids.
While Griselbrand isn't nearly as expensive as he once was, I'm sure Magic players will always be happy to open copies of the Demon in their Modern Masters 2017 packs!