"Why does an ordinary American citizen need an assault rifle?"
Well, for one, I am an extraordinary American. Americans have led the free world in many endeavors, and to call one an "ordinary American" is to immediately denigrate the accomplishments of Americans over the centuries, present Administration excepted.
Why do I "need" something, well, I don't need the tobacco I use in my pipe, and lung cancer kills quite a few people every years. I don't need the beer in my refrigerator, as as Randy Travis can now attest, (naked DUI?), abuse of alcohol can lead to many unpleasant incidents, some of those including death. I abuse neither, and use each sparingly. But I don't need them. So when are the "for the children" types going to ban them - they aren't Constitutionally protected, like firearms are, (SCOTUS, Heller vs US and McDonald vs Chicago), so when will they take these killers away, for the children?
What? It's been tried and failed? Gee, darn, too bad.
Since my firearms ARE Constitutionally protected, a level much higher than my pipe tobacco or Bud Light Platinum, two items which for which I have no requirement to prove a "need" for, and two items that have not been successfully removed from American society despite the level of death and injury misuse of them has caused, then I would have to say I likely have no need to prove a "need" for my firearms, either. You see, they don't "need" me to be unarmed. If they do, as paraphrased from Arfin, "what horrors do you have planned for me that you "need" me to be defenseless?"