There were no standing armies at the time that the constitution was written. If the nation went to war, civilians were called in to form a militia and they were expected to have their own weapons.

Reading many of the papers that the 2nd amendment was based on you would find that one of the concerns of the founders was a tyrannical government taking control. They felt that citizens should be able to defend themselves in such a case.

In later years, the Supreme Court has ruled that the right to bear arms extends to all citizens for their own personal protection.