The structure of the epistolary novel consists on documents that are brought up by the main "writer" of such documents. These can be letters, diary entries, journal entries, or personal observations. The idea behind the epistolary novel is  a) to inform the reader about the author's personal journey in a specific event, or  b) to add more realism to a story by providing "first person accounts", or primary sources that show that what the story is telling is actually true, or has deeper validity. 

In So Long a  Letter, the format follows the structure of, literally, a very long letter in which Ramatoulaye expresses herself as a woman, as a wife, as a mother, and as an individual.

 To love one another! If only each partner could move sincerely towards the other! If each could only melt into the other! If each would only accept the other's qualities instead of listing his faults! If each could only correct bad habits without harping on about them!

She is practically venting out in the letter about how she feels as a widow, but also after her husband, a Muslim, took a second wife, leaving Ramatoulaye alone with 12 kids to fend for herself. 

Yet, aside from venting, Ramatoulaye analyzes her gender, in general. This, as a result, brings the empowerment that many women in male-dominated societies seek for, in order to understand their own identities. 

Eternal woman: even in mourning, you want to make a strike, you want to seduce, you want to arouse interest!

Aside from her identity, Ramatoylaye also realizes the complications of love, the craziness, in general, that is expected of relationships, and the dissonance between our inner worlds and the world outside that seeks for us to follow rules, make amends, understand one another...and yet, we must also follow our hearts. She also defines friendship, and how it fortifies one through difficulties; how it is resilient even in times when couples separate, and (in all) how friendship surpases romantic love in general.

L'amitié a des grandeurs inconnues de l'amour. Elle se fortifie dans les difficultés, alors que les contraintes massacrent l'amour. Elle résiste au temps qui laisse et désunit les couples. Elle a des élévations inconnues de l'amour.

All of these observations, made by a woman who is otherwise treated as a second class citizen in her society,  definitely shows an amazing amount of empowerment as well as a huge hunger for finding her role, her place in society as an empowered woman. This is the main gist of epistolary novels when the format is applied to characters seeking to find validation: they can serve as conduits for self-analysis, much similar to literary works that use stream of consciousness.