Other mistake: When Newt gives Jacob a suitcase full of eggs, he bumps into him, apparently swapping suitcases. If you watch carefully, the suitcases don't actually switch.
Other mistake: A shot of Jacob and Queenie walking was used twice. Queenie is showed on the left side of the screen and Jacob on the right, and Jacob looks behind, then turns to look to his left, then turns more in front of himself. This happens while Queenie and Jacob are trying to find Newt's case, then find what cell Newt and Tina are in when they are trying not to be killed by Grindlewald in disguise.
Chosen answer: Obscurials often manifest around childhood. Children are young and influential, so Graves/Grindelwald can persuade one to let their magic free without much protest in a public place as an indestructible weapon. As we witnessed when Credence rampaged through New York, Obscurials would be far more destructive to whole cities than just casting a lot of spells.
There's WAY more to it than that-although you'd never know it if you only ever watched the original HP films and never read the books. In short, Graves/Grindelwald has had first-hand experience with an Obscurial before. Remember that painting Aberforth Dumbledore had on the wall of the Hog's Head Pub in Deathly Hallows Part 2? That was Ariana Dumbledore, his and Albus' sister. When she was a child, she was attacked by three Muggle boys who caught her doing magic, which traumatized her and caused her to become an Obscurial; the Obscurus inside her later killed the siblings' mother. At the same time, Dumbledore and Grindelwald were plotting together to bring wizards out of hiding and reassert their superiority over Muggles with the help of the Elder Wand. In the end, Aberforth confronted them, leading to a chaotic duel that resulted in Ariana being killed by a stray curse. Dumbledore and Grindelwald went their separate ways, with Grindelwald putting their visions of wizard supremacy into action, while Dumbledore's grief for his sister led him to repent and shun power for the rest of his life. Virtually none of this made it into the movies. But that's why Grindelwald wanted an Obscurial so badly-because he'd seen one before and knew first-hand how lethal they could be.
Actually, it was never 100% confirmed that Ariana did become an Obscurial.