Cynically, I'm sure it could be "justified" in all kinds of ways like "knowing more about the photos viewed most often with our Photos app helps us to determine

Cynically, I'm sure it could be "justified" in all kinds of ways like "knowing more about the photos viewed most often with our Photos app helps us to determine what new features would best enhance the user experience. Only metadata is being collected, so no employee is ever able to view your pictures. We value your privacy blah blah blah" Even more cynically, it just gives them more data about you and your interests (with viewing frequency/duration being a measure of interest) which lets them push more narrowly targeted ads at you, which gives them more money, which lets them invest more in improving Windows as a whole or whatever else they tell themselves so they can face their own reflections.