Well, yes, that's kind of the point: given the ability to deliver a explosive to your enemy's position, what makes it worthwhile to include (generally expensive and inconvenient to tranport) chemical weapon material rather than additional explosive and maybe some shrapnel. If it's that the area remains dangerous longer, then congrats, you have impaired your own ability to maneuver into or through that area once the enemy retreats.
WWII shows the absolutely mind boggling amount of bombs you can drop on an area without actually killing that many people. After buildings have already been hit once they make repeated strikes less effective. Chemical weapons are pound for pound vastly more deadly, especially as a follow up and more importantly disable people tying up resources for medical treatment and evacuation.
The most recent example is https://en.wikipedia.org/wiki/Ghouta_chemical_attack which killed or incapacitated ~5,000 with a few rockets. It also acted at least in the short term as area denial which can be extremely useful.
PS: In terms of fighting a near parity army, simply using tents separated by sandbag walls makes bombs or rocket strikes significantly less effective. Add even the threat of chemical weapons and things become significantly more difficult. http://armymomstrong.com/wp-content/uploads/2010/02/2-milita...
