Even minor characters do much to advance the plot in short stories such as "How I Met My Husband" by Alice Munro. In this story, the two aforementioned characters, Loretta Bird and Mrs. Peebles, both serve as a source of information for the main character, Edie. They help to relay important details both to the main character and to the reader as those details become necessary. For example, it is Loretta Bird who announces that the airplane that arrived has come to stay for a little while and that the pilot will be taking people on rides.
However, this is not the only reason that these particular characters are important. "How I Met My Husband" is a story that intensely focuses on femininity, as the main character is a young woman who has a brief but memorable romance with an airplane pilot who, ultimately, leaves her behind. The main character is only fifteen when this happens, but both Loretta Bird and Mrs. Peebles are older women and therefore more worldly than this young character.
They both serve as contrasts to the young and innocent Edie, who does not know much about romance or even understand what it means to be "intimate" with a man. This is evident in the climactic scene in which they ask Edie whether she has been intimate with the airplane pilot and she responds that she has, though they only kissed, to which both women react with astonishment.
Overall, it is seemingly minor characters such as these who serve to enrich stories, providing the reader with more background, more thematic elements to consider, and more ways to engage with the reading material.