This type of mark is longer than a hyphen and can be used to replace parenthesis, commas, colons, semicolons, etc. It's more of a stylistic choice.

EXAMPLE: Billy Bob Martin—the owner of the sandwich shop—doesn't like cats.

In this case, the em dash replaces the commas.

EXAMPLE: Sally Sue has two parrots—she likes birds.

In this case, the em dash replaces the semicolon.

I'd recommend limiting how much punctuation the em dashes replace because if you replace all commas, parentheses, and semicolons with the em dash, its usage may become confusing/distracting.