In "The Godfather", when Kay visits the compound to track down Micheal, there is a shot up car, which Tom Hagen explains had been in an accident and that no one was injured. This was before Sonny's incident. Is there any information about why this scene was put in ?
The movie script was created based on the book. In the book, and also in the movie, it is implied that there was some tension regarding the families and that security was pretty tight.
The bullet ridden car implied that there was an assassination attempt on some member of the family. That's mostly Vito Corleone or Sonny. It signaled to the audience and represented the fact that the family knew about the assassination. That was the reason Sonny and Vito were heavily protected.
Please note that while the movie script is based on the book, Mario Puzo did make some changes in the movie for continuity.