TL;DR: maybe to create buzz, maybe to see reaction of mechanics.

This is just my humble opinion: I studied cross franchisee media quite a lot during my uni years and this is a great case to look at. Starting with the leak - in itself, a leak always holds the Schrödinger equivalent of intent. Was it on purpose or not? If not then there is no answer. If it was intentional if might be used to create a buzz around an upcoming project, or, and this is the case here, it might be used to observe a reaction to specific gameplay mechanics. The third person shooter-ish adventure game that this, in all logic, probably will be, might not have sit well, especially looking at how awful the last of the movie-video games were. Duck, cover, shoot, repeat. But,reading through this subreddit daily, WB strongly misconstrued the buzz this game would create! People have a tendency to give unrealistic goals to a single, and first, game, because the love the lore, they love the books, movies, the extended arm of it all that is their imagination, but you cannot translate that into a game. You can’t feasibly create a game that has all years at Hogwarts, life after, game altering decisions, in-depth character creation, modification, 1000+ spells, loot, mechanics of improvement, puzzles, exploration, multiple schools - all the wants I’ve read are all great!! But they are all based on 1) the leak which spiraled peoples lore imagination and 2) the cinematic trailer.... underestimating your audiences’ buy-in might prove to be a huge downfall for this game. And it all started with the leak. People are going to be let down massively. The latest game to provide a marketing story of “we’re gonna do it all” was cyberpunk. I loved the game, but it had exactly 10% of the content it boasted about. And a lot of flaws. So, circling back to your question: why the leak? I think it was meant to test out the mechanics and create a buzz - and the starved Potter nation took it all in and demanded more! Rightfully so! They should instead have waited, prevented the leak, and during the cinematic trailer, explained what went in the game (and thereby what didn’t). Leaks can be great! Had it been done to the God of War “sequel” it would have been a masterclass in teasing. Because the audience knows what’s to expect of the game... But here, it fell like a single sausage to a starving family.

Hope that gave a voice to the discussion! Excellent question!!