[url00]

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.)

[zvr]

Microsoft recently introduced typing via Windows key plus hyphen (en dash) and plus shift (em dash). Much easier.

[thfuran]

I think offsetting the em dash with spaces is contrary to almost every style guide.

[jachee]

Chicago does it one way and AP does it the other.

I definitely prefer it un-spaced, though, personally.

[thfuran]

MLA and APA as well as Chicago.

[oneeyedpigeon]

I think this is a localisation issue; in the UK, it's much more common to see space-surrounded em dashes.

[thfuran]

As far as I know, the UK styles that recommend spaced dashes are using en dashes where US English would use em dashes, and the ones that actually use em dashes, like Cambridge or Oxford, are unspaced.