It's mobile, for one. You can cover and threaten a big area with a drone, which means a tiny amount of chemicals go a long way, and that solves the other main issue with chemical weapons.
Main goal would be to sap morale: a few grenade-sized bombing runs and you force everybody to wear chemsuits under the desert sun.
Chemical weapons are particularly difficult to use en masse outside of enclosed places like a subway.
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).
I think so, actually. The most effective places to set up a bomb are probably among the most dangerous for a covert operative to actually get to. This changes that.