Dumbledore had hoped that Newt would be able to save Credence, thus preventing Grindelwald’s vision. In classic Dumbledore fashion, he didn’t explain any of this to Newt beforehand, but better late than never, I guess.
Article source: https://www.cinemablend.com/news/2467049/fantastic-beasts-reveals-key-grindelwald-and-dumbledore-history-in-deleted-scene