bottom line / ˈbɑːtəm laɪn /

It is a well proven best practice to end long writings with a short summary that wraps up our message in a few lines. It is not only a matter of structure, but also a way to ensure good communication and that the main points are remembered.

CNET reviews' summaries
always include the good, the bad,
and the bottom line

When I studied English, I learned that the most common ways to close such kind of writing forms would probably start with something like "In conclusion", "In short", "Summarizing", but I don't recall having ever used "bottom line". And yet it is so frequently used.

The "bottom line" is the key message, the short sentence you want your audience to remember and, therefore, very similar to "conclusion" (conclusion would be the end of your reasoning thread).

However, "bottom line" is not only used at the end of writings. In speech, it is also used when the speaker has extended a bit long about something and wants to summarize it in a sentence. But let's see it in a few examples.

This Week in Photography 226: Retro Active (38:00)

If you are and old school shooter who is used to having an aperture ring in your lens then you may actually still desire to have a lens that has an aperture ring. [Shooters] jumping in the photo world today, he's not going to cry over the lack a lot of the buttons and dials that I feel really comfortable driving in my Canon gear today. So, ultimately the bottom line here, I don't care whether is photographers and cameras or a chef and a knife who is julienning a carrot, there is a point at which you become so good in whatever tool you are using that it truly does become an extension of your thought

Ok, despite the "bottom line" I got lost too, and I'm not sure if this is the most descriptive example, but it is a recent one, and one that illustrates its use in speech.

Let's see one in writting.

The Online Photographer: The Pentax Q System

Bottom line: you want to stop down very far to get increased depth of field or slow shutter speeds? Then you're going to sacrifice peak resolution.

And another one:

New York Times: Martin Scorsese’s Magical ‘Hugo’ (interview)

[Martin Scorsese] The bottom line with “Hugo” is it’s a story about the boy and his relationship to his dead father. It’s more serious than funny.

So I think that it is a quite convenient expression, isn't it?