Because we used them a lot, everyone carried at least 2 of them. Radio operators carried more because the order to pop smoke would most likely come over the radio. They were used for directing choppers to land, or boats to beach, or to let the battalion CO in the air know where we were. They were also good for letting the gunships know where to shoot.
They came in red, green, yellow, and purple and the smell is something that you don't forget. I connect that sulfur smell with the sound of chopper blades and 100% humidity and 100+ heat.