They had to defend themselves against what they considered tyranny and they wanted to ensure that for future generations should the need arise
Answer:
At that time in revolutionary America militias comprised a large part of the military network to respond for contingency operations. The 2nd amendment seems to have been developed to protect the right to maintain a defensive posture against further invasion, such as from the British or French, when a standing army could not necessarily muster or mobilize quickly enough to respond to developing threats at every border, or within.
