Weight training is an effective way of getting your body into better shape, losing weight and gaining muscle tone.
Weight training increases your metabolic rate, causing more calories to be burned than before weight training. Your muscle mass will increase, which will then cause your metabolic rate to increase still further.
"Strength training prevents and fights osteoporosis. Studies have shown that exercising at least three times a week improves bone strength, and lowers the risk of osteoporosis. Regular exercising also slows down bone loss. Weight lifting, aerobics, jogging and stair climbing are few activities that increases load on the bone and stress the bone are helpful."
You can reduce the risk of injury when you have stronger muscles and tendons from working out. Your blood pressure and LDL cholesterol levels can decrease, which can reduce your risk of heart disease. You can increase insulin sensitivity which will also decrease your chance of developing diabetes.
http://fitho.in/2010/07/23/why-weight-training-benefits