The organization served as a vast internal affairs division that maintained as well as monitored relations between the Star Command and the Romulan fleets. They serve a critical role during wartime where they ensured that the chain of command was maintained through their officers. This prevented Commander's from disobeying orders as the Legate's ensured that commands were followed to the letter. Thus, its officers often monitored all activities within the fleets and reported suspicious or unusual activities to the Star Command.
Whilst serving as a way of policing the ranks of the military, the Tal Diann also served as an intelligence branch for the Romulan military. They did not maintain the same level of power as the Tal Shiar but commanded a great level of respect from the military. It was known that there was a level of competition between the Military Affairs Division of the Tal Shiar and the Tal Diann with both sides trying to bring about the political destruction or downfall of the other. (Last Unicorn RPG module: The Way of D'era: The Romulan Star Empire)
In 1411, the scientific arm of the Tal Diann discovered the means to isolate and harness the power of a quantum singularity to serve as an energy source for the Romulan starship fleet. This allowed the Star Empire to venture out beyond their solar system and become an interstellar power. (Decipher RPG module: Player's Guide)