IMO citizens being well armed is deterrence. It makes the likelihood of 'evil martial law' government less likely by increasing the perceived cost. If you truly disarmed the entire country (something I consider basically impossible), it makes a military coup/governance much more feasible. One of the main lessons of modern history is that insurgencies are incredibly cost effective ways to fight a stronger opponent. You basically have to be willing to commit terrible crimes against humanity to overcome them.
Of course this is all theorizing. I don't America is in danger of that sort of civil war just yet.
The moment anybody tried to organise armed resistance to the government, they would instantly be declared “terrorists” and taken out by legal means or by force. The US government agencies have such information supremacy that it is nearly impossible for any armed groups to organise without the government knowing about it.
Of course this is all theorizing. I don't America is in danger of that sort of civil war just yet.