Occasionally the editors -- inevitably based in Chennai -- spot a typo that the reviewers missed. Sometimes they cock things up massively, especially equations -- I had a big argument about a tickz-based scheme once. The formatting is done automatically and for me as a latex author I really think we could do that very easily. Colleagues who use word see the text transfer to something better as a major value add. The publicity the journal adds is a strong function of how "good" it is. Science and Nature effectively exist because they are Science and Nature.