"What legal need do you have for a Corvette, mansion, or expensive food? A VW bug, apartment, and PB&J's get the job done."
It's the principle. Government does not exist to determine what an upstanding citizen can have. Government exists to protect its citizens. From that purpose we derive the (very sound) logical basis that weapons intended for a conflict between nations (WMD's, heavy weapons/vehicles, etc) should be relegated to national control via the military. An automatic weapon does not meet this criterion.
Instead of asking why you *need* something, ask this: why should an upstanding, law-abiding citizen be denied something outside the aforementioned weapons?