Frank Sinatra is admired by professional singers because he was a master of phrasing.  In music, phrasing divides lyrics into natural segments, creates connections and a natural flow and conveys meaning through emphasis and pauses.

In writing, phrasing has a counterpart called rhythm.  It helps create a unique voice, a melody and a flow of meaning.  Why is it important?  Understanding rhythm will improve your writing overnight.  Guaranteed.

In a 1985 book, writing coach Gary Provost created this tour de force to demonstrate what happens when the writer experiments with sentences of different lengths.

This sentence has five words.  Here are five more words.  Five-word sentences are fine.  But several together become monotonous.  Listen to what is happening.  The writing is getting boring.  The sound of it drones.  It's like a stuck record.  The ear demands some variety.

Did you feel yourself getting tired?  How closely does it resemble writing you see every day?  Here's the next paragraph.  Listen for the rhythm:

Now listen.  I vary the sentence length, and I create music.  Music.  The writing sings.  It has a pleasant rhythm, a lilt, a harmony.  I use short sentences.  And I use sentences of medium length.

Can you hear the difference?  Can you hear a melody, a rhythm?

And sometimes, when I am certain the reader is rested, I will engage him with a sentence of considerable length, a sentence that burns with energy and builds with all the impetus of a crescendo, the roll of the drums, the crash of the cymbals –– sounds that say listen to this, it is important.

That was one sentence, far more compelling than "This sentence has five words."  The passage ends with Gary's advice to writers

So write with a combination of short, medium, and long sentences.

Create a sound that pleases the reader's ear.

Don't just write words. 

Write music.