By requiring real names, Facebook is trying to combat the sort of social discord that occurs on sites like Reddit where people routinely abandon identities, or create throwaway identities to participate in specific fora.
Imagine if car license plates had our name and home address on them, would we be as likely to commit road rage, pick our noses, double park, etc.
Facebook sees itself as a commons first, and the product vision seems to be to incorporate as many of the kinds of constraints common in meatspace to nudge specific kinds of behaviors and discourage others.
Recall in its origin the biggest difference between Facebook and Myspace was Facebook's calming blue, un-customizable color scheme. Blue is commonly used in police uniforms and hospital scrubs.
Facebook also reinvented the feed, moving very early on to an algorithmic feed when competitors were still under the impression that people just wanted to see all updates from all friends. Facebook very quickly understood that the right algorithm can turn a boring feed into engagement dopamine.
Consider that unlike a purely online forum, posts on Facebook are much more likely connected to people we met first in real life where our status is already well-established. Nonetheless people use Facebook to signal activities, pictures or beliefs that they feel will increase their status.
When normal meatspace dynamics matter so much to a site's content, allowing fake names pretty much breaks voyeurism, which is a feature, not a bug.
It is precisely the dark pattern of being able to look up anyone on Facebook and peer into their life that creates Facebook's opportunity to arbitrage between the feeling that what we are posting is mundane, yet it being far from mundane when assembled by Facebook into a complete dossier for anyone who wishes to look us up.
But these are side-effects. My interpretation of this is that Zuck has a strong conviction that meatspace and online life should fully merge. I think he believes this as part of the same utopian conviction that led him to envision Facebook as a "social utility", and I think the same one that is leading him on a path to holding public office.
In his ideal world, politicians would not be two-faced, and internet users would not have multiple avatars and identities to flippantly switch between to avoid facing up to reality or to explore a part of themselves that they do not put on display for all to see.
I think the question Zuck must also ask himself in his moments of reflection is "How can Facebook truly do good?". The good that needs to be done in the world needs to be done by (and for) real people, not screen names and throwaways. Thus as building blocks of that utopian edifice, we must all bear the weight of social responsibility that goes with our actual name being attached to everything we do.