Frequently, writers must describe their works in a nutshell. For example, when seeking an agent or publisher for a novel, the writer sends a query letter. In the space of a single page, the query must describe your book's plot, main characters and central conflict. It must also convey your style, show off your credits, and tell the recipient why he should spend months of his life marketing your masterpiece.
Jacket copy is another form of nutshell description. When I browse through a book store, I read the jacket copy to find out what each book is about. The writer has to grab me in a paragraph or two.
Writers of essays and magazine articles have to write pithy pitches, too. They've got to sell that baby within the space of an email so short the editor won't have to scroll to read it all.
Although everyone has to write them, I'm not sure query letters are the best way to screen the good from the bad. There has to be some way, though, otherwise agents and publishers and their poor assistants would have to read through 500 novels to find one worth publishing (at least, maybe, after a long string of edits). I speak from experience.
Some people are good at query letters. But someone who's a genius at expressing herself in a 350-page novel can be clumsy at spitting it out in a single page. Too bad, pal! I've heard famous authors say it was easier to write the whole novel than the itty bitty query letter.
Then there are loglines. A logline conveys your entire story in a single sentence--the same nut in a smaller shell. Loglines are used a lot for screenplays, but every writer needs to be able to use them. When someone asks, "What's your book about?" you'd better spit it out, and it had better not take all day.
Think of it: you've spent X number of years squeezing your blood onto the page to tell the story in your heart. Now, describe that to me in a nutshell.
And you'd better get it right.