It's a bit like 2 space indention versus 4 or 8 space. I find the em-dash example pretty compelling because visually relaxes my eye more than commas do — similar to how prefer at least 4 space indention over 2.
(Unrelated: The alt-code for em-dash on Windows is Alt+0151. I just typed my first manual em-dash wow exciting.)
I think offsetting the em dash with spaces is contrary to almost every style guide.
Microsoft recently introduced typing via Windows key plus hyphen (en dash) and plus shift (em dash). Much easier.