Why a laser?
Simple: Because in space, no one can hear you scream. ;-)
If you don't get it, then you've never thought about the velocities and distances space warfare will occur. And yes, while we are not yet at the point of cheap economical space travel, space based weapons are an issue today and will become more of an issue tomorrow.
On a more positive note, this might also be used as a tool to clean up space debris, which too is a very dangerous problem. Ok, maybe not this chemical laser, but I also believe that there is work on a solid state laser too? ...
With respect to the use of a laser on earthbound targets... You have a laser traveling at the speed of light, so the target is always in the line of sight. (No need to lead the target, no need to worry about wind drift ...) Of course not a cheap way to kill a target but that will change in the future.