punctuation - When to use the em dash (—) in fiction writing?

I always see the em dash (—) in fiction writing.

Here are some example from Haruki Murakami's After Dark:

Komugi comes in and takes something from one of the cartons piled against the wall—a fresh bathrobe to replace the one from room 404.

Mari says, "She told me he took everything—her handbag, her money, her cellphone."

"Well, she couldn't help it," says Komugi. "When it starts, it starts—bang!"

When to use the em dash (—) in fiction writing?