I've seen many kits of and references to smoke dischargers and know they are supposed to be used for obscuring purposes. But I wondered when and how they are typically employed on a battlefield.

On modern US tanks, we were trained to use them when we detected inbound enemy missile fire. Most anti tank guided missiles required a gunner to continuously track the target and guide the missile until it strikes.

The smoke, followed by a rapid zig zag maneuver by the tank (called a "Sagger Drill") was an attempt to obscure your tank from the enemy gunner and make it harder for him to hit you. Also a liberal spraying of machine gun fire in the direction of the missile's backblast was an attempt to give the gunner something to think about other than tracking his missile.

They are mainly used when you need to conceal your position or move. Hence they are often used for defensive purposes.
Some grenades are true anti-personnel ones and not only smoke grenades. The former are used for close protection.