If you don't need one that's fine and understandable and hopefully a conclusion you came to on your own and not one that's been force fed to you through media or by your government.
1 reason you need one is because it's inline with today's standards. An assault weapon is just a gun with a magazine, stock, pistol grip. There's a reason police stopped using revolvers and pump shotguns (although units still use shotguns) and switched to semi automatic handguns and semi automatic rifles.
The cops have them, the military has them, criminals have them, why shouldnt you? You never know when you have to stand against any of them.
#2 reason is it's a slippery slope. First they'll ban assault weapons, then it'll be things like bump stocks, red dots, scopes, lights, braces, trigger modifications, suppressor, then there will be more magazine restrictions only 10rd, only 5rd, then it'll be ammunition restrictions only a certain grain, no hollow points and eventually they'll raise the age of buying shotguns and rifles from 18 to a mandatory 21 like handguns, then they'll expand the restrictions on ownership from felons, people with assault or drug charges to other things that'll restrict people from legally owning. Then there will be city bans on ownership aka if you live in this city you cannot legally possess a firearm and it'll likely target inner city minorities whereas more affluent areas which are often driving distance may have free reign to ownership.
What it all boils down to is that if you let them they will disarm black people and kill them using the same weapons they disarmed them of.
We literally just had a black man Auhmoud Abery chased down and killed by white men with guns. And those white men are walking free. He didn't have a gun, he didn't have drugs or possess stolen property. shyt philando castille was a legal carrying citizen killed in his car. They don't think we (black people) should have guns and they don't want us to have them and some black people are more than happy to throw away their right toward ownership as if it makes them any less of a potential target or victim, meanwhile everyone else wants to defend their right and in some cases expand them.