Chemical warfare is fucking great when you intend to move in after - traditional bombs are too messy and destroy too much property.
The logic is similar to Dead Kennedys' take on the neutron bomb in their aptly named song Kill the Poor:
Efficiency and progress is ours once more
Now that we have the neutron bomb
It's nice and quick and clean and gets things done
Away with excess enemy
But no less value to property
No sense in war but perfect sense at home