The vehicle for voice-over established right at the beginning of this movie is prayer. Prayer is very often internal, happening in a person’s head, so establishing a character’s voice-over as their prayers or talking to God (or any kind of deity or idol) is a great way to make voice-over natural.  

The other component here is angels looking down at the main character George Bailey’s experiences on earth and giving commentary. By depicting them as stars, the script establishes that these heavenly voices are angels. Then, when the heavenly voices comment on the action, we know where they are talking from and why they are omniscient.