In a well edited movie, there are no scenes that don't either move the story ahead or explain something not otherwise known about the protagonist or his/her story.
In John Wick (2014), there is a specific scene I would like a better explanation for, as to why it was left in the movie.
What does the cop scene which happens after the end of the first assassination attempt at Wick's house provide for the viewers?
As an explanation of the scene, shortly after Iosef reports back to his father about the car he stole, and was chastised, his father sends an assassin squad after Wick, to Wick's home. We see the squad taken care of one by one, by John Wick, in a fashion which reinforces the father's exposition of John as "baba yaga," or the boogy man. After the final assassin is killed in Wick's front hall, the door bell rings. Wick looks up and sees (as we see, from his point of view), a silhouette of what looks like a man in a bulky jacket and hat, back lit by blue and red flashing lights. Perhaps a Police Officer.
John Wick answers the door, gun hidden behind his back to find that, yes, it is an officer standing there. The two of them greet each other, cordially, on a first-name basis. During the friendly dialog, the officer asks Wick if he's "working again" while glancing at the dead man over Wick's shoulder. Wick replies, "no, just sortin' some stuff out". To which the officer finally replies "ah well... I'll leave you be then" and turns and returns to his squad car.