Flak guns were designed in the year of 2341, six years after the Great War. Initially, it was used as bomb interception screen, which was a success. Later, it turned out to be effective against fighter and bombers.
They rather focus on combat against fighters than warships. Even though, flak guns are also effective against other smaller-sized ships such as transports, freighters or even weaker cruisers. When a projectile of flak explodes, it has a short damage radius, which makes it ideal against fighters and especially bombers.
Flak projectiles usually do more damage on hull plating than on shields. Flak guns are also known to be nearly useless against subsystems.