The necessity for the Founders: they were not allowed a voice or any representation in government.
The Confederacy was in precisely the opposite situation.
"Necessity" is in the eye of the beholder. It is arrogant of you to tell me how much pain I must endure before I decide it is necessary to do something about it.
The southern states felt it necessary. That’s all that matters.
You don’t get to be the judge of what is or isn’t necessary for other people. That is up to them to determine.