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.

It gives rhythm and flavor to your writing. Varied punctuation - where appropriate - is an easy way to spice anything up, give it a little more flavor, more control over how your words are read. There is a quote by Gary Provost that isn't specifically about punctuation but illustrates this point well, it's one of my favorites:

This sentence has five words. Here are five more words. Five-word sentences are fine. But several together become monotonous. Listen to what is happening. The writing is getting boring. The sound of it drones. It’s like a stuck record. The ear demands some variety. Now listen. I vary the sentence length, and I create music. Music. The writing sings. It has a pleasant rhythm, a lilt, a harmony. I use short sentences. And I use sentences of medium length. And sometimes, when I am certain the reader is rested, I will engage him with a sentence of considerable length, a sentence that burns with energy and builds with all the impetus of a crescendo, the roll of the drums, the crash of the cymbals–sounds that say listen to this, it is important