The Color Purple is stylistically an epistolary novel, written principally as a series of letters between major characters. This is important to the novel because the letters include aspects of dialect and educational background that are important to understanding character and setting. One can tell the difference between Celie's writing and Nettie's, for instance: Celie uses dialect more heavily, while Nettie, probably because she has reached a higher education level, uses dialect more sparingly.
In the following passage from Letter 55, Nettie writes to Celie in Standard American English (SAE), demonstrating her level of education in relation to Celie. Despite this difference, Nettie never talks down to her sister, speaking from a place of empathy and kindness rather than elitism:
I hadn’t realized I was so ignorant, Celie. The little I knew about my own self wouldn’t have filled a thimble! And to think Miss Beasley always said I was the smartest child she ever taught! But one thing I do thank her for, for teaching me to learn for myself, by reading and studying and writing a clear hand.
While Nettie likely still speaks in dialect, she writes in perfect Standard American English. Celie both writes and speaks in African American English (AAE). This sisterly juxtaposition is important for both women's characterization in the novel and could not be achieved as effectively without the use of epistolary style.