Making Long Stories Short

What does make a long story short mean?

Where does this phrase come from?

According to Cambridge Dictionary used when you do not tell all of the details. 

https://dictionary.cambridge.org/us/dictionary/english/long-story-short

When do we tell stories like this? Why do we use them? Who do we usually tell these types of stories to? Why are they useful? 

Example: To make the long story short, I got fired.

When and why would this story long story be appropriate to tell?

Long Story Short IMage 1 - August 16 202

How do you make a long story short and effecient? What important plot points do we choose to help get our story's point across?