https://www.snitchseeker.com/harry-potter-news/david-yates-eddie-redmayne-dan-fogler-talk-fantastic-beasts-plot-points-obscurus-106934/
SnitchSeeker: Why did Grindelwald) want the Obscurus and control of Credence? What did he hope to do with that dark force?
David Yates: Its very unusual for that magic to exist, and it’s a very powerful piece of magic, in a sense – an Obscurus. He was keen to weaponize it. To use that dark force to deal with one of his enemies, basically. So his intent was to weaponize it. That was the agenda. And in the next stories we discover more about what he intends to do with that extraordinary power.