Questdat, I understand why you question the function of this particular scene. As is so often the case, the novel gives us a better, more detailed account of what happened. Not only did Johnny rattle off his list of grievances as mentioned in the film, Johnny tells Vito 1) that he gave his ex-wife and kids more than the court ordered in financial support, 2) he hit his current wife after allowing her to cheat on him, 3) and that made propoganda films against the advice of Vito and which angered Woltz.

In the film, there are only about :20 - :30 for Johnny to account for his troubles. Regardless, I think this exchange is two-fold: 1) to demonstrate Vito's power and 2) to show that Vito will defend those who show him respect, loyalty, or who have blood-ties.

What really worked well in the novel that was impossible to put in the film was when Puzo asked how each of Vito's sons would have reacted to the reproach Vito gave to Johnny. Johnny and Hagen both chuckled. Puzo speculated that Sonny would have rebelled and stormed out, Fredo would have cowered to such a tongue-lashing, and Michael would have remained quiet and not be heard from for weeks.