On one hand, it will tend to give large projects with established communities a centralized location to handle questions which otherwise would be in a StackOverflow tag. Perhaps core developers would be interested in the community management features that Discussions offer, which would be harder to do in a world where questions get delegated to SO.
And potentially linking to the actual version/release that fixes a question would allow maintainers to whittle out the stale/deprecated answers that can bring down the average quality on SO.
On the other hand, it seems like small/medium size projects might suffer from a brain drain effect; if most React experts stop/reduce checking SO, and start/increase checking github.com/facebook/react/discussions, then there's probably less chance at the margin of a React expert seeing something tagged in SO as React,SmallReactLib.
How do folks think this is going to pan out?