Proper posture and alignment are important because both affect the way you move and carry your body and weight. Alignment and posture both play a role in balance. Learning proper alignment and to appreciate how it affects your posture can help you on a day to day basis e.g. while sitting, standing, and walking.
Proper posture helps you to decrease the abnormal wearing of joint surfaces that could result in arthritis and keep your bones and joints in the correct alignment so that muscles are being used properly.