A common question today is, "Why does anybody NEED an assault weapon?"
Translation the misleading 'assault weapon' into 'modern sporting rifle' such as the AR15, a reasonable answer would be:
For defense of my home, it's wise to plan for the worst event most likely to occur. Therefore I would prepare for an unwanted entry by multiple persons.
In order to meet that requirement, I would NEED: A firearm that is lightweight, manuverable, and dependable. It must be small enough to use in the confines of a room, but long enough to have natural point-and-shoot ergonomics. The weapon must be low-powered and low-recoil to assure good control when shooting repeatedly quickly and repeatedly.