Light tanks were designed with speed in mind. Usually of short silhouette and little armor, they were used for reconnaissance and hit & run attacks. They are essential in an army that wants to be well informed of enemy movements and positions.
Light tanks lack the firepower and armor to deal with most other tanks effectively. They should, for those reasons, be restricted to anti-infantry support, hunting down Trucks and giving intel to the Platoon Commander. They are excellent infantry support vehicles due to their quick reload speed and multiple machine-guns. High mobility means they can relocate quickly, making hit & run attacks highly disruptive to the enemy.
They can also be used inside urban areas, although, it is advised against.