What is the significance of Mrs. Mitty's presence in The Secret Life of Walter Mitty?

The Mittys do not have a dysfunctional marriage but a marriage that is symbiotic. They really need each other. Walter Mitty is so introverted and so absent-minded that he would find it hard to take care of himself. He would not be living in a rural setting in Connecticut if he were single. He would be living in a Manhattan apartment or residential hotel. Mrs. Mitty is an anchor. She makes all the important decisions, but she is dependent on her husband in many ways. These people are typical of the Manhattanites who were starting an exodus to outlying regions because the city was becoming too stressful, too dangerous, too expensive, too crowded, too noisy, and a lot of other things. Real estate in the surrounding country was cheap before and for a while after World War II. Automobiles were becoming more dependable and roads were becoming better. It is appropriate that "The Secret Life of Walter Mitty" opens with the couple traveling in a car. Automobiles were reshaping America.

Mrs. Mitty depends on her husband to take her into Waterbury for her weekly hair appointments and shopping errands. The overt action in the story all takes place because of her requirements. Mitty is a chauffeur. He spends much of his time just waiting. He waits for her in the hotel lobby, and he is waiting for her at the end when he imagines that he is waiting to get shot by a firing squad.

Mrs. Mitty is useful for storytelling purposes. She makes it possible for Thurber to convey information through dialogue rather than straight prose exposition. She creates some drama through her bickering with her husband. She gives the reader a perspective on Walter Mitty which might be missed if we only knew him through his fantasies. The story would be less interesting if it were mainly told as an interior monologue. The minor ongoing conflict between husband and wife makes the story dramatic. 

"The Secret Life of Walter Mitty" seems autobiographical. James Thurber was married twice. He was introverted and eccentric, and he capitalized on his eccentricities by working them into stories, essays, and highly unusual cartoons which made him one of the most popular humorists of all time. One of his favorite subjects is the relations between married couples.