So microsoft will release windows on all form factors (pocketable, tablet, netbook, notebook, desktop, servers, etc) and they will have a single appstore that handles all their software distribution.

One can purchase a license for, say, ms office on their desktop and when they use a tablet, she can login to MS' appstore from the tablet and downloads her copy of MS Office for ARM on the tablet.

It expands the playing field for them, their customers and their partners/developers.