You missed the idea. He was discussing that even complicated programs (and games) can be converted to use Windows 10 features (i.e. modern desktop) flawlessly without breaking existing code. He wasn't talking about replacing Steam.

Why would it divide your user base? Isn't it being used as a glorified package manager? Everything is still Win32 under the hood.

Well, since the game is no longer married to Steam at that point, my users would be losing Steam's achievements, mods and matchmaking. Am I not understanding that right?

They showed off installing mods from Steamworks in Age of Empires II during the demo.

Those were mods from SteamWorks? Sorry, I didn't catch that part. I still genuinely wonder why I would want to convert my app into a "modern desktop app".

    Sandboxed security. 
    Easier install/uninstall.
    Backup/restore.
    Live tiles and notifications
    Downloadable from the Windows Store (which means it will show up in Cortana search results when searching for it, even if it's not installed)
    Ability to use excellent new WinRT API's
    Deep linking
    Share to other apps easily (e.g., post to Facebook)

etc.

Thanks.

To make is discoverable by non steam users?

To have a second, larger, market. Window userbase is huge, much bigger than Steam's.