Good posture helps with sporting performance as special positions are often crucial to success, eg the position throughout the golf swing.
People with good posture also feel better about themselves. An upright body position is often a sign of self confidence. People who are less confident will sometimes show this in their body language, for example by adopting a slouched posture.
Regular training tones muscles and helps to create good posture. In addition, muscles will hypertrophy (increase in size) and develop better endurance.
Muscle tone developed by regular exercise makes daily tasks such as shopping and gardening easier. It also helps to prevent injury as good posture reduces the strain on muscles, tendons and ligaments.