You have to remember that, at the time, the Colonies weren't really all that well developed. People NEEDED guns. Indian raids, dangerous wildlife, and just regular ol' crime was a threat. Furthermore, North America had a troublesome quality about it in that it was colonized by two major European powers: England and France. And unfortunately, these two rarely saw eye-to-eye.
The French and Indian Wars, which took place just prior to the Revolutionary War and in which many of the Founding Fathers fought, was won in no small part by the colonists themselves, who provided their own weaponry. These people gave sweat, blood, and tears for their King.
In return, they were met with crippling taxes and economic sanctions over the next few decades. Don't get me wrong, wars are expensive, and the British Empire, rich as it was, needed to recoup some funds. The problem was that the Colonies were not consulted in ANY of these proceedings. They were British, they spoke British, they came from Britain. And yet they were being treated like the subjects of conquered nations.
Furthermore, once the British realized that revolt was inevitable, their very first action was to attempt to seize the arms of local Revolutionaries. No guns = no war.
>tl;dr
Guns were a necessary part of life back then. The guns of the average, private citizen was indispensable in protecting their country. And the Founding Fathers realized that the first action of a government which fears its own citizens is to attempt to disarm them.
There are many practical reasons to own firearms, and the Founding Fathers took those as a given. But the Second Amendment was written *specifically* to ensure that the armed citizenry would always be there to protect themselves, their family, their friends, and their countrymen against whatever they might face. Even if that enemy was their own government.
