Chemical weapons provide more of a terrorizing effect than raw kills. It demoralizes civilians and it harasses the military. They can also have long term negative effects on survivors as well.
I am reminded of the "Black Smoke" from various [cross-media] versions of "War Of The Worlds". Granted that an alien species may not fully want, need, or utilize native architecture but keeping such intact at least for initial reasons just after successful invasion has uses.
The benefit would be to kill the population of a city without destroying the infrastructure.
The moderm stuff is sticky and persists even with rain and sun for months, supposedly. It's terrifying.