Microsoft's OneDrive spots your mates, remembers their faces, and won't forget easily
(www.theregister.com)
what reason would anyone have
That's none of your business. You don't need to know why anyone wants to do a thing to advocate for their freedom to do it.
Just because you lack the imagination to think of reasons someone might have, doesn't mean that they don't have a perfectly good reason. But, they shouldn't need to justify themselves to you.