Its because they know that drinking lots of pure water is essential for clear, healthy looking skin. Water hydrates the skin and prevents dryness, it also flushes impurities from the skin and prevents breakouts.
Beautiful skin needs good nutrition.
People who eat a spectacular diet of whole grains, organic fruits and vegetables, and multi-colored salads tend to have naturally great skin and slim bodies. They could probably get away without taking a supplement. Other people who live on junk food are likely deficient in a lot of nutrients, they have acne from the hydrogenated oils in chips and chocolate bars and they tend to look heavy and unpolished. Most of us are somewhere in the middle, we try to eat our veggies and salads, indulge in the occasional pizza and tend to have a few pimples and dimples. To make the most of what the goddess gave you, feed your body the right fuel and avoid like the plague anything fried in cheap oils.