[gruez]

What advantages does a user account have over anonymous session ids? User accounts survive users clearing cookies, but I doubt users clear their cookies often enough for that to be a problem.

[marcinzm]

Mobile browsers wipe cookies, users switch devices, multiple computers, etc.

Facebook has over a decade of data on its users, no session id will match that.

edit: Also knowing users emails will allows advertisers to match the reddit accounts to their own data stores on users.