This exercise will help you strengthen those upper back muscles and restore mobility to your thoracic spine (upper back). In addition, doing this exercise with your hips back as far as possible will flatten your low back so those overused muscles can't contribute.
Keep in mind that this exercise isn't a big "strength" exercise where you need to add more resistance by piling weights onto your head (I really don't recommend that!).
Add the exercise to your warm up, get your brain to learn how to fire up those upper back muscles, and then they'll be easier to connect to during the rest of your workout, when you're at the desk, or in the car.
Think about how strong you'll look and feel with good posture and a tall spine.
Doug Barsanti
ReInvention Fitness
P.S. Your posture is often the result of habits - make them good ones.