Another reason is that it changes a persons relationship to pain. (Self-Mortification Video)
Another is that trance can be used to stop you from having second thoughts or changing your mind in the middle of battle.
Trance and possession can be used to instill fear in others, which has all sorts of uses in fighting.
But just getting your opponent to space out for a split second is often enough to run them through. For some reason, perhaps it is the spooky music or the strange distortions-- this video gave me a fright. I don't know what is happening. I've watched a bunch of these and often the movements of the dance and the clothing or head-dresses cause me to space out for a second.