It's not really chemical weapons it's White phosphorus or "Willy Pete's"
Usually they are used to mark a target or to destory equipment.. However sometimes if there isn't a hard target to hit they are used to hit troops.
The US has used them in Iraq as well. It's not a banned weapon by the UN so they are still used. Same with DU.. eventually they may be banned but not for now it's still fair game.
http://en.wikipedia.org/wiki/White_phosphorus