Here's why:
* Metaphor: "Bottom line" literally refers to the last line on a financial statement. It's used metaphorically to represent the most important or essential point, often in a business or financial context.
* Idiom: The phrase has become so common and understood that it is now considered an idiomatic expression. It's no longer directly linked to the literal "bottom line" on a document.
You can use "bottom line" to emphasize the most significant takeaway, summarize the core argument, or highlight the practical outcome of a situation.