memes

Em dashes often replace parentheses:

The company has a policy of having any newly hired employee (like Steve) introduce themselves at the monthly all hands meeting.

The company has a policy of having any newly hired employee—like Steve—introduce themselves at the monthly all hands meeting.

Em dashes also often replace colons:

I'm going to bring my signature dish: bacon-wrapped dates.

I'm going to bring my signature dish—bacon-wrapped dates.

Em dashes are commonly used to denote interrupted speech:

He started to explain, "I was hungry and you weren't home yet so I—"

"You're not a diabetic, you can handle waiting a few extra minutes to eat"

Replacing commas is unusual and probably incorrect according to most style guides.

This is also highly localized. Style guides tend to apply only to one particular country, not all English-speaking countries. The AP guide is used by most American newspapers and magazines, and the Chicago Guide is used by most American book publishers. Each have their own rules on dashes.