Why does the point of view matter in the story of Scarlett ibis?

The first person point of view is used to emphasize the theme, tone and mood of the story. One theme of the story is the narrator's regret over his pride and selfishness in his attitude toward his brother. The story begins with the narrator thinking back with a feeling of regret to the circumstances surrounding his brother Doodle's life and death. In the story he admits to this selfishness and pride. Since it is his own admission it carries more weight than if a neutral observer gave an opinion that the narrator had acted out of selfish motives.

Both the tone and the mood are rueful, regretful and guilt laden. These could be conveyed to the reader only by the person who had those feelings.