yeah, krlea got it. It was showing how Michael never agreed with what the family was into. Sonny was mocking the people that were in the war and then michael informs them all that had just enlisted in the marines. It showed how he had diffent beliefs than everyone else in the family and how he never really wanted to be a party of family business which you can also see by him not getting up when Vito arrives and everyone else at the table runs to the door to greet him. FFC probably decided to put this scene in there at the end to show how everything that Michael had done since he shot Sollozo and McCluskey was one whole big thing that he wished he had never gotten into. He had to kill his own brother! After he did that, I think it made him realize and remember why he had never wanted to get involved with it in the first place.