It made sense when America was getting started and had no army, navy (or air force for that matter).
Every man would own his own gun to help protect his country from foreign powers and his state from other states.
That was the point of the second amendment.
Now that America has a sizeable army, navy and air force, it makes less sense.
I doubt you'll hear "quick everybody, North Korea is attacking, every man grab your gun." Except in the movies - that is.