It was so USA could have a well armed militia
This citizen army was to be used to protect against tyrannical government; foreign or domestic. Consider the times when it was written. The last thing the citizenry wanted was another "king" and among the most popular reasons for a continuance of this right (it was a long established law in england) was to deter a tyrannical government. At the point of adoption that would likely have been one of the chief reasons the citizenry championed it. Other reasons would certainly be to fight off invasion, to protect your property/self defense, to have an armed police force etc. But to suggest that the 2nd amendment allows for militias only to act on behalf of the federal government and not in order for the citizens to have the ability to hold their government to account is misleading imo.