Advantages of nourishing the skin from within
Every cell in the human body needs dozens and dozens of nutrients and metabolites. Some, like vitamins, minerals and essential amino acids need to come from food. Others are produced by the body provided it is healthy and properly nourished. No skin cream can replace that.
Applying a cream with nutrients to the surface of you skin does not ensure that those nutrients actually penetrate into your skin cells. They may just "sit there" until your next shower. How much of the active ingredients actually get into your skin cells depends on the skin's condition, concentration of the ingredients, manufacturing technology and many other factors. This doesn't mean that all topical preparations are useless -- but they are often unreliable. On the other hand, when the nutrients are ingested and absorbed into your bloodstream, they are sure to be delivered to your skin cells.
Nutrition has some effect on the mechanisms of aging of the body as a whole. Inhibiting these mechanisms slows down the overall aging process, including the aging of the skin.
Nutritients and foods that benefit your skin also tend to benefit other body systems and overall health.