Why was i prescribed valium for ptsd?

Not sure: I can't speak for your prescriber. But benzodiazapines are sometimes ordered as they can "numb" the pain. Ptsd in often a chronic condition and long term use of benzodiazapines can lead to tolerance in some people.

Ask your doctor: It's best to ask the physician who prescribed this, as there are no rigid algorithms of what to prescribe for ptsd. Valium is inexpensive and can be both a muscle relaxant and calm anxiety; muscle tension and anxiety can easily be part of ptsd. Valium can also be habit-forming, though, and is usually not the first thing most psychiatrists would prescribe. Hopefully you're also in therapy?