It is certainly open to interpretation as to why Updike uses Sammy as the narrator in his short story "A & P." This is my perception.
This often anthologized short story is generally seen as a coming of age or "initiation tale." In keeping with this theme, by the story's end Sammy realizes that (in some way) this incident and his actions mark a turning point in his life.
Sammy is important as a narrator because he has yet to see the world from a wider lens as almost everyone eventually must when they "grow up." At his age, the world is about today: it's summertime and he's working at the supermarket with a guy who is married but still acts like a kid. He really has nothing to worry about.
When "Queenie" enters the store, everything seems to stop: like it only can in a movie...or in the mind of a teenage boy. She is a welcomed diversion from his tedious world of canned goods and housewives.
Sammy and Stokesie joke for a minute, but then Sammy is amused to see Stokesie come around and get in touch with his "responsible married man" side, as he attempts to act like the store manager he hopes one day to be.
Sammy is young and inexperienced. It appears that what he does is based on principle. However, he also wants to impress the girls.
...I say "I quit" to Lengel quick enough for them to hear, hoping they'll stop and watch me, their unsuspected hero. They keep right on going...
It is perhaps for this reason that Sammy's character is appealing to readers of all ages. Like many kids his age might, he sees the manager, Lengel, as an inflexible sort—the enemy without a heart. He describes the older man as "pretty dreary, teaches Sunday school and the rest..." This seems more insult than praise...at least from someone Sammy's age.
Sammy's parents see the entrance of the girls as "the sad part of the story..." However, Sammy does not agree: he cannot see what his parents see. This is very much evident when he reflects upon Lengel's discussion of the store's rules:
"It's our policy." He turns his back. That's policy for you. Policy is what the kingpins want. What the others want is juvenile delinquency.
Sammy's statement literally separates the men from the boys. In his eyes, it is all about the establishment and the "delinquents." Sammy cannot imagine what it is like for Lengel who has to answer to his bosses. If Lengel wants to keep his job, he does not have the luxury to wave the rules for the cute teenagers that come in bathing suits; it is safe to assume that at one point in his life, he probably saw things as Sammy does but those days are long past. Lengel knows what he must do as manager of the story, mired in responsibility when one cannot quit a job so casually.
Lengel cautions him, noting that Sammy may not be sure of what he is saying. Sammy acknowledges that Lengel may see it that way, but Sammy says he does know what he is doing.
Lengel, a friend of his parents, tells Sammy not to do this to his parents. Sammy has an uncomfortable moment because he does not want to do anything to upset his folks. At the same time, his pride is on the line:
But it seems to me that once you begin a gesture it's fatal not to go through with it.
Sammy makes a clean exit.
Sammy's perception of what he is leaving behind and what lies ahead can be found in his description of Lengel and his own response as he glances through the store's window:
I could see Lengel in my place in the slot, checking the sheep through. His face was dark and gray and his back stiff...and my stomach kind of fell as I felt how hard the world was going to be to me hereafter.
Had Lengel been the narrator, the tale would have been much different. However, Updike uses Sammy as the narrator—his inexperience with the world and his youth are what allow him to unthinkingly brush the summer job aside. However, Sammy intuitively realizes (it would seem) that he, too, will someday be in Lengel's place, and that is not an easy place to be.