These primary breathing muscles need space and freedom to perform! When their movement is cramped and crowded you get less and less oxygen. Less oxygen makes you feel more and more like slumping. A lose-lose situation.
Good posture produces the greatest amount of oxygen while spending the least amount of energy to get it. Good posture translates as bone balanced lightly upon bone, with as few muscles as possible engaged in the support of that balance.
A slump is actually tiring since your muscles have to remain slightly contracted to keep you from falling face down into your minestrone. Remember that you head weighs from 12 to 20 pounds. If it is pushed forward off your bony spine then it is your poor tired neck and shoulder muscles that have to hang onto it, hour after hour after hour. Headache? No surprise.
Be well. Breathe beautifully.