Conventional chemical weapons are not cost-effective, because you have to hit a big area and cause big effects for it to be worthwhile, since both delivery and payload are expensive. With drones you can shoot/drop grenade-sized payloads that hits just a few people, barely more effective than an actual grenade. The goal is to sap morale and force lopsided reactions (such as forcing the opfor to wear chemsuits, or invest in cleanup teams).