Bradbury uses characters in "The Martian" purely as information tools. He has Williamson provide some useful info in "The Long Years." The store owner in "The Luggage Store" is useful because of the background he provides. Even Hathaway, who is the main character in "The Long Years," primarily serves to tell us that the Martians died from chickenpox in "—And The Moon Be Still As Bright."
Bradbury needs all of these information tools because he has so much to explain about Mars and what people are doing there. Notice that most of the regular interchapters don't have any information tools; the narrator does that work.