To remind us of how Michael was when he was younger. How he tried to distance himself from his family and in the end that's how he ended up, without his family. Remember how he resented his father and Tom for discussing his future?
We also learned that Sonny introduced Carlo and Connie, which explains why he felt so responsible for looking after her (not only b/c she was his little sister but also b/c they were together b/c of him.)