Here is a link to an excellent article on posture. The author explains the following:
The Benefits of Good Posture
Makes You Look Taller. Good posture adds an inch or two to your height. Try it, you will be surprised.
Makes You Look More confident. Good posture gives you an air of strength and confidence. Think about it… shoulders back, chest out, chin in….
Improves Organ Function. Slouching forces your rib cage to compress your organs decreasing their efficiency.
Reduces Tension and Pain in Your Neck, Shoulders, and Back. Most of the discomfort you get from sitting at work comes from sitting improperly.
Increases Concentration and Mental Performance.
Prevents Beer Belly. The slouch accentuates any belly fat you have while proper posture hides it well.