Microsoft has opened an app store called "Microsoft Apps" inside Google Play store. This way - Microsoft seems to think - it's easier for Android users to find apps made for Android by Microsoft. All apps are in one place, not here and there in Play store.
Android users might not be that interested in Microsoft apps since there are many alternative apps to choose from. However derived Android ROMs with no access to Play store would benefit from Microsoft apps if that option was available. It remains to be seen if Microsoft will launch a store outside Google Play store.