It's secure, its apparently faster on some points and its cheaper licensing. Oh, and according to the demo, its easier to administer via a special tool.
That's probably about the sum. Mostly useful for students, certain corporations or government departments, libraries and the like. People with simple app needs.
Although I expect the store will grow with both "s" and "windows on arm", both via centennial bridge, and via true UWP, but you can't very well choose an OS based on its future prospects in the present. So basically you'd have to be happy with what they have already, office 365, Spotify, iTunes and more yet unknown centennial ports and new uwps (which may potentially not include ones you want, if you want more than what is known)