When would you use something like this -- in a sentence?

Question

I have seen it in sentences before and I should want to know the rule behind using it. If anyone knows what I mean.

*want to know, not should

Answer 1

It is called an "em dash."

"The em dash is used in much the way a colon or set of parentheses is used: it can show an abrupt change in thought or be used where a period is too strong and a comma too weak. Em dashes are sometimes used in lists or definitions, but that is a style guide issue; a colon should be used instead."

Answer 2

The em dash (—), also known as the em rule, indicates a sudden break in thought—a parenthetical statement like this one—or an open range (such as "John Doe, 1987—"). The em dash is used in much the way a colon or set of parentheses is used: it can show an abrupt change in thought or be used where a period is too strong and a comma too weak. Em dashes are sometimes used in lists or definitions, but that is a style guide issue; a colon should be used instead.

got it from here

Answer 3

in a sentence tell me what you like about your friends?

Answer 4

sometimes -- is used to further explain the things you are making a point of.