To me, the dash says the writer is really going to give me something extra, an afterthought, or an insider secret—and who doesn’t love secrets.
There are threes time when you can use the dash:
1. As an abrupt break in the sentence.
Dressed in the stolen gown, she danced around the castle—momentarily, forgetting the scent of death that followed her every step—and begged the dark prince to dance with her. (Here you see, the words between the dashes completely change the image that the reader might otherwise envision.)
His first thought on getting out of bed—if he had any thought at all—was to get back in again. (I didn’t write this sentence, but I still like it—anyway, in this sentence the writer is giving an added dimension to the character. We read the words between the dashes, and we think, perhaps, this guy is not the sharpest tool in the shed.
“I’m not going to clean the house—you’re not my mother—and I am going out with Raunchy Randy.” (Your stepdaughter might give you a sentence like this. Whahaha!)
2. As a long appositive:
He lifted the weapon—a sawed off Remington 12 gauge—pointed it at the crowd and fired. (Remember, an appositive is renaming the last noun used. The words between the dashes tell you exactly what kind of weapon and it also adds a big dimension to the kind of damage the weapon can do.)
3. As a long summary:
He struggled though the murky waters, infested with deadly creatures as small as a leach and as large as a crocodile; everything that slithered, gurgled or bit was here in this black pool—a primordial soup of despair for all animals borne of land that barred his escape. (In this sentence, I’m basically re-telling you what I already told you. Note: If you’re going to repeat yourself, you’d better make it good.)
In the book ‘Writing Well’ the legendary Donald Hall tells us: Use dashes with caution—but use them.
I say: Have fun. Experiment, but don’t use them too much. Dashes are something you save for special occasions. I tend to have a lot of special occasions—so sue me. Call the grammar police. Whahah!