To fight depression effectively one must not only reduce the symptoms of depression but also any conditions or syndromes causing these symptoms. Addressing only the symptoms is to try to fight depression with one hand tied behind your back. Depression can be caused by chronic stress, a recent loss, a genetic predisposition, a brain injury, as well as an overwhelming, traumatic experience. In the case of post traumatic stress syndrome symptoms, resolving the frozen, sketchy, emotionally-laden memories can fight depression at the roots.
So it does help to know the cause or causes of depression. But that does not mean that the symptoms themselves shouldn’t be treated while the causes are being investigated. If you are blowing your nose and sneezing so much that you can’t drive to the pharmacy, then your cold symptoms should be treated even before the cause is known. It is the same when someone has to fight depression.