An em dash or long dash is a very handy punctuation mark.
The em dash (—) sets off a word or clause and adds emphasis. Or, it can signal an interruption or expansion of an idea. It’s also the longest of the dashes—hyphen or en dash.
An em dash is the width of the letter M, or two dashes when typed.
Space should not be used before or after the dash. Some publishers however may opt to use space before and after the dash. The essential thing is to retain consistency throughout a manuscript.
An em dash should be used sparingly in formal writing. But when used correctly, it can make a sentence clearer and easier to understand.
It is particularly useful in a sentence that is long and complex or in one that contains a number of commas, as in the example:
I bought shirts, sweaters, jackets, mufflers, and caps—all of which were on sale, of course—for my family to use in the winters.
Em dashes can be used in place of commas, colons, semi-colons, or parentheses. They are mainly used to set a phrase apart.
In place of commas:
Some examples, with and without the em dash to highlight the value of using one.
You are my friend, my only friend, who cares about me.
You are my friend—my only friend—who cares about me.
Can you please listen to what I have to say…oh, nevermind
Can you please listen to what I have to say—oh, nevermind.
In place of colons:
I know where I want to party tonight: the new pub down in the main street.
I know where I want to party tonight—the new pub down in the main street.
In place of semi-colons:
I like listening to her speeches; she gives a lot of helpful advice.
I like listening to her speeches—she gives a lot of helpful advice.
In place of parentheses:
She finally spoke (after thinking for two minutes) that she did not understand what he had said.
She finally spoke—after thinking for two minutes—that she did not understand what he had said.
An em dashes is a great tool to use when a writer needs to add additional information in sentences.
However, it must be used sparingly. If used too often it can complicate the flow of sentences, leading to confusion.
To read more on Em Dash please visit
https://www.grammarly.com/blog/why-you-should-love-the-em-dash/
https://www.uis.edu/cas/thelearninghub/writing/handouts/grammar-mechanics-and-style/dashes-parentheses/
https://www.thepunctuationguide.com/em-dash.html
https://www.punctuationmatters.com/hyphen-dash-n-dash-and-m-dash/