I don't know how things look from microsoft's point of view, but I see lots of "nice to have" aspects of being in charge of one of the popular social platforms.
Lots of people cut their programmatic teeth building simple things that interact with these platforms. It would be nice for Microsoft if every teenager building a bot to play fart noises on discord does so on Azure™ building with Microsoft Visual Studio Code™. The fact that AWS has been the standard "go to" place for people to get cloud servers up and running has been nice for them.
Also, it creates opportunities for these "cool" companies to write technical blog posts for your stuff. I think Discord has helped Elixir rise in popularity - I'm sure Microsoft would like to see something like their "how Discord scaled to 5m concurrent users" for C# (or whatever stack they want to promote).
Now, I have no idea how big a deal this would be. I really doubt it's a $10b big deal. I agree that "the community" isn't a reliable factor. If anything, I think this purchase puts the community at risk. People like that Discord is a small company that needs to care about how good its servers are. If they suddenly are owned by a giant company that might care less, people might look around for alternatives.