They probably have a unique pacing. Because the story is revealed bit by bit through the written exchanges, it can create a slow - burn kind of effect. This can make the reader more engaged as they anticipate what will be revealed next in the letters. Also, the use of different perspectives in the letters can add depth to the story, as we get to see the same events from multiple points of view of the letter - writing characters.
Most of these books would likely use the epistolary format to explore complex relationships. Whether it's a friendship, a romantic relationship, or a family bond, the written communication between the characters allows for a detailed exploration of the emotions, conflicts, and growth within those relationships. For example, a letter might reveal a character's hidden jealousy or their new - found understanding of another character, which are elements that make these novels so interesting.