•Why it earns a slot
Why it earns a slot: it fuses three separate treatises, on death, on the gods, on the state, into one work, and book V admits it was written to talk himself through grief he calls being "so severely exercised by fortune," turning philosophy from a classroom exercise into a survival tool.
Cicero stages five days of dialogue proving virtue alone can face down death, grief, and fear, then closes with a Roman general's dream vision of the afterlife reserved for those who serve the state.
This distillation is written from the freely available original, which is always the better read when you have the time: Project Gutenberg.