Now, read the comments provided by Ryan. Other than a post I ran in 2008 on Why the Gun is Civilisation, this is one of the best explications of the question "why?" With due credit to Ryan, and hoping that he would give his permission if I were to ask, I will quote his entire comment here:
Overall, not funny or accurate and a bit offensive. I own a few guns, each tailored for a specific purpose, but I'm not an NRA member, nor a member of any other gun rights/gun control organization. I feel that your understanding of those who vigorously support the 2nd Amendment is woefully lacking, and you approach it as a "I see no use for guns, therefore no one else has any use for them" issue. If you haven't been given a "reason," then it's either because you don't accept their reason as being valid or you simply aren't really trying to find an answer--because you already have 'your' answer. Regardless, I'll give you some reasons, since you feel that simply having the right to do something isn't good enough.
As a human, I know better than to place full trust in other humans. When the going gets tough, there are very few people that you can trust not to look out for themselves first, at the expense of those around them. This applies to any situation, be it political or a real survival scenario. Even in the best of times, we still have humans preying on other humans, either for profit or for pleasure. In the worst of times, such as after a natural disaster or in lawless regions, this human behavior of looking out for yourself and "to hell with everyone else" is amplified. In either of these times, those charged with keeping order (law enforcement) are rarely, if ever, there when the chaos starts, and the first minute/hour/day is left solely up to the victims and the perpetrators. In that short window, you can have everything taken from you by people who don't care about you, but for themselves. Your family, your home, your life, gone to satisfy the desires of someone else.
But what if you could do something about it? What if simply by making it known that you will not be a victim, you could avoid all that heartache, pain, and loss? Doesn't your family look to you for protection? Are you not their provider, their caretaker? Isn't it your responsibility, your duty, to protect those that you love and those that depend on you to live? If you don't think so, then you should probably tell you wife and children that you won't try to help them, and that they should just wait for 911 to show up. At least that way, they won't be under the disillusion that their father actually cares enough to protect them, and they should have a backup plan.
If you do want to protect them, then how best to do it? Naturally, you avoid dangerous situations as much as possible, do everything you can to not make yourself a target, but what if it does happen? Do you call the police, hide, and hope for the best? Or will you be a parent and defend that which you hold most precious? Most importantly, HOW will you defend? What could possibly put you on the same level as one, two, three large male attackers, possibly armed? If only you had something that was easily accessible, could be held in the palm of your hand, and could be pointed at the threat to make it go away. You might not even have to use it--just letting the threat know it may sustain significant injury or loss of life by attempting to victimize you could deter it.
Luckily, such things do exist. Handguns level the playing field. It doesn't matter if your young, old, male, female, built, or scrawny, you will still be able to defend yourself with a handgun at a second's notice, so long as you know how to use your firearm and you practice. Simply displaying a handgun often makes would-be attacks decide to find a softer target. It's called a Show of Force, and it's a principle that's used world-wide by every civilization. And if the threat persists, then it does so at its own peril. This isn't to say that handguns aren't abused and used improperly. When people willing to break the law are banned from having them, they will frequently find ways of obtaining them and they will use them as tools to aid their criminal ways. When people who want to stay right with the law are banned, they remain defenseless and become victims. We've seen this everywhere such a ban has been enacted.
It doesn't have to be guns, Bob. The guns are just a means to an end. It could be a device that lets you set people on fire with your mind, or pill that you could take that let you fling people across the room with telekinesis, or a damn board with a nail in it. It doesn't matter, so long as it's the most effective means for me to defend myself and my family, and ensure that they are given the best shot at life that I can provide for them. As it happens, guns are the most effective means ("expressly designed to kill humans as efficiently as possible") that we have today. You know it, I know it, and all of our politicians, Democrats, Independents, and Republicans who are packing heat know it. Maybe in 100 years someone will be having this argument over the right to keep and bear photon cannons, but it will still be the same exact principle.
I'm loathe to get into "assault rifles," because first I have to explain to you the difference between that term and the "assault weapons" term that you're confusing it with. Civilian ownership of "assault rifles" is prohibited, unless you get permission and a permit from the federal government, which you have to renew regularly. It's not easy. "Assault weapons," on the other hand, such as the AR-15's most people are familiar with and even the civilian model of the AK-47 are not the same as assault rifles, as they are not "select fire weapons." They fire in semi-automatic, one shot per pull of the trigger, only. Real AK-47's fire in single shot/fully automatic, and M-16's (the military equivalent of the AR-15) can fire in single shot, 3-round burst, or full auto. You can't go down to the gun store and buy any weapon that functions like that. The Assault Weapons category also includes modern handguns that hold over 10 rounds and shotguns that can adjust their butt-stocks to properly fit your arm length. For reference, I own a rifle, a shotgun, and a pistol that, for one reason or another, all classify as an "assault weapon." Now that that's out of the way, we can move on to the reason.
The rifles in question are the most technologically advanced firearms that fulfill nearly every role conceivable. A semi-automatic rifle can be used to do everything a hunting rifle can, plus be useful for any form of tactical situation at any time of day against any type of target, with the correct ammo. With my semi-auto chambered in .308 I can take down a buck to feed my family or protect my land/house/country from any human threat short of an armored vehicle. The semi-automatic rifle such as the AR-10/15 is a versatile firearm that can generally be outfitted with any number of attachments, making it useful in daylight, darkness, poor weather, or harsh terrain. A magazine capacity of 20 or more rounds ensures proper redundancy for any situation, regardless if you need one round to drop a deer at 500 yards, three rounds to drop an intruder at 2.5 yards (ideally I'd be using my shotgun or handgun, however, for my neighbor's sake), or 20 rounds to keep the mob from looting your house and burning it to the ground. Bottom line: it is efficient and useful in any situation. It's BETTER than a handgun, because it's more accurate, holds more ammo, and generally has more power. The only drawback is that it isn't easily accessible like a handgun.
Those are my direct, honest, human answers, Bob. Not nearly as succinct as "because I can," but I also had more time to answer than the people you may have asked. I own these weapons, not because I want to use them, but because I'm prepared use them protect my family, myself, and my fellow man against the realities of this world using the most effective means possible. End of story.
I'm not going to go into all of the reasons our fore fathers felt we needed firearms, most notably of which was to keep our government in check, because that would be my 'political' reasons for owning such weapons, not my "human" reasons. Needless to say, they are just as lengthy.