If you are shortsighted with your training goals, you will more than likely fall into the trap of training just for vanity.  In other words, primarily training just the muscles that you can see in the mirror.  The main muscles of youth, however, are found on the backside of the body.  These muscles will be there for you when you are 70, 80 or even 90 years old and plus, they will make you look more powerful!