When a law abiding person who exercises discretion, reason, compassion, and an aversion to force goes about their lives and finds themselves and/or their families in an attacker’s grasp (in public or at home), the only thing they can pick is their response to the attack, be that attack an attempt at rape, assault, kidnapping, or murder. If it is already too late to avoid the attacker, the remaining option is to defend your own life with a firearm.
Criminals, despite their depictions in TV and movies, are not dumb. They know the benefits of strength in numbers. Most home invasions and a growing-number of muggings involve between 3 and 5 attackers. Recent cases have shown that 5 or 6 rounds might be needed to stop one attacker, which would require the people defending themselves to have between 15 and 30 rounds. Since the attackers will not be kind enough to wait for you to reload, that all needs to be in one magazine.
The most commonly available firearm that meets that requirement is a semi-automatic rifle, comes from the factory with a 30-round magazine. Since such rifles are not practical to carry to the grocery store or other public venues, a semi-automatic handgun, which commonly comes with a 12- to 15-round magazine, is a decent second alternative.
According to the FBI, violent crimes have been falling throughout the US for the last 20 years. 2011 reported 14,600 homicides (down from 23,700 in 1992), with just over half of them being committed with a firearm. At the same time, the number of gun sales has been continuing to grow. While exact statistics on sales are not available, the FBI reports that in that 20-year span, 130 million firearm background checks have been run.
As firearm ownership has gone up, crime has gone down. While I do not presume to say that one caused the other, it is also difficult to say that an increase in firearm ownership causes an increase in crime.