Movies Anywhere is backed by the major film studios -- Warner Bros., Disney, Universal, Sony and Fox -- as a way to centralize the far-flung films of theirs that you've purchased around the internet. Films produced by those studios appear in the Movies Anywhere app, regardless of whether you bought them through Apple, Amazon or (as of today) Microsoft.
By connecting your Microsoft account with an account for Movies Anywhere, you'll have access to all eligible movies bought via Microsoft to watch on non-Microsoft devices, and you'll be able to watch films you purchased on other participating services on your Xbox or Windows 10 gadgets.
The company is also running a promotion that comps you a copy of X-Men Days of Future Past.
But don't confuse Movies Anywhere with a subscription service like Netflix or Hulu. Movies Anywhere simplifies the stuff you already own, but it doesn't unlock any libraries other than your own.
Corrected at 2:55 pm PT on Aug. 7: Fixes the spelling of the service in the headline and last paragraph.