Many people feared that the new Constitutional powers the federal congress would have over the Militias (to provide for organizing, arming, & disciplining them) could actually be used to make them LESS effective, either through in-action or disarming, quite contrary to the stated purposes of & in the Constitution.
"...Congress could take such measures with respect to a militia, as to make a standing army necessary. Whenever Governments mean to invade the rights and liberties of the people, they always attempt to destroy the militia, in order to raise an army upon their ruins." Mr. Gerry, 1st Congress
By declaring that well-regulated (well-trained to arms) militia were NECESSARY, i.e. mandatory, it would "furnish a greater certainty of its being done". By securing the right to keep AND bear arms of the people - who would serve in the Militias - it would be assured the govt couldn't DISarm the Militias, or otherwise render them ineffective (i.e. by declaring who could serve), and so reduce the need for that bane of liberty - a large standing army.
The Militias were made from the body of the people, whose duty it was to serve; they better have the right & means to do so, and should NOT be dependent on the...generosity of the federal govt for them.
Now there were various people who were/would be exempt from Militia duty by law. It hardly seems plausible the FF expected them (themselves) would give up a right to personal arms simply because of that. So that right doesn't seem to be dependent on the 2nd's obvious militia purposes for its existence.
Of course the militia purposes are barely viable any more, not since the decline and recreation of the Constitutional militias into the federally controlled/armed National Guard, Yes 'the people' are handed a bone as part of the "unorganized militia", but that is hardly "well-regulated". (and shows why things like AWBs and mag limits are found to be constitutional vs the 2nd & their ideal use for a militia).
= new reply since forum marked as read