I qualify posture as a habit and, therefore, trainable. By training yourself to keep a good posture your body gets vertically aligned and the position is sustained by muscles, bones and joints the way they were intent by mother nature and ensures proper breathing, minimal stress on the joints, balanced load on your muscles.
By not keeping a good posture (keeping a bad posture, slouching) you compress the front of the body and make breathing more inefficient (you take shorter breaths), the joints are overstressed and slowly but surely damaged and the core muscles are unevenly used and they will either atrophy or overdevelop.
The benefits of a good posture are both physical ( good breathing, well developed muscles – if you ever wondered an ass qualified as “cute” is one consequence) and psychological ( decreased anxiety level , increased confidence).
A bad posture generates long-term pains: joint pains – knees, muscle pains – lower back, all the physical consequences of bad oxygenation – including … worsened snoring.