When it does happen, hackers will be able to decrypt any vulnerably encrypted messages they capture now. We need to switch to post-quantum encryption well before quantum computers that can break encryption actually are built, such that any data they do actually decrypt is old enough it’s worthless.

But the real reason why Microsoft is doing this now is simply because the decision on which algorithms should be the first official standard for post-quantum encryption have just been finalized. Expect a lot of companies to be adding post-quantum encryption in the next couple years.