I want to find the political/cultural reasoning for having a Regular Army. An Indication of Permanence and like the Constitution a mirror of order and legitimacy?
2 commends.
1. Scotts army in the war against Mexico was about 1/3 regulars and 2/3 militia (volunteers).
And this was the plan to do again when the federals first mobilized in early 1861.
As long as the main small arms where smoothbore muskets (or if the NRA/volunteer rifle movement had existed earlier) that solution do make sense... when you don't want an army based on mandatory military service or having a full professional army. (costly and there is the "English" distrust of a standing army)
This give you a good sized force that you can trust to do the hard jobs. Flanking (or frontal assaults and rear guards.)
And the volunteers/militia give you the extra manpower needed for the defensive jobs and simple biding the enemy in front.
And as others mention, war became more technical. In Denmark all the privates in the infantry where men who where in the process of or had done their mandatory military duty and then returned to civilian life.
But in the artillery and engineers you did have a good number of professional soldiers who had enlisted for a number of years... and in wartime they would then be reinforced by privates from civilian life.
So in the CSA having at least part of the artillery and engineers be made-up of professionals would be in line with how things was done in other armies.
2. yes legitimacy. If we go back to Europe in the 18th century, the army and navy where usually the main expenses of the state/king... other than the royal court. And after mercenaries where replaced by standing armies "owned" by the state/king. then having an army was a central part of being recognized as a sovereign political power.
(that is one of the reasons why the American "states" are not allowed armies and navies. Having this on the federal level is a important part of having the US be respected as a sovereign political power internationally. And why the states are not recognized as such internationally)