Turrets are the basic weapon used by ships. They are capable of dealing with small threats alone, or large unshielded ships enmasse.
Unlike weapon systems, turrets can target hostile creatures and vehicles. So they are very useful when assulting cities as troops can fire at defenders from relative safety in the sky. However the limited range and power of the turrets limit their ability to engage ships or heavily armored targets.
Turret damage and range does improve with TL.
They are the only weapon currently available that can pierce through power armor effectively