It's very important to stay hydrated from the inside. Your skin constantly loses water so it's important to stay hydrated. Also when your lips are dry and chapped, it could be a symptom of dehydration. Drinking plenty of water should help you to get a soft and smooth skin
Drinking enough water can also help to keep your hair moistured. If your body is losing a lot of water and you don't drink enough to keep the hydration level up, how is your body supposed to do its job? It will take care of what's most important to stay alive first, and you don't need hair to survive ;).
Of course we don't all have to be as thin as Keira Knightley, but I'm pretty sure there are a lot of girls who'd want to lose some weight. Therefore it's extremely important to drink lots of water. It tends to reduce hunger, raises your metabolism and it flushes down toxins and by-products of fat breakdown.
Sometimes your body can't tell whether you're hungry or thirsty. You might think you're hungry but in fact you really just need to hydrate yourself. Especially if you're craving for something salty, you might need to hydrate your body, as salt holds onto water in your body. Try drinking a glass of water instead.
Also, if you drink a glass of water before every meal, you will probably eat less because you'll feel full a lot quicker. Eating an apple before your meal also helps a lot as it gives you a fuller feeling.
Raising the metabolism
Drinking water raises your metabolism because it helps in digestion. Fibers are also very important in this part!
Flushing down toxins
When consuming food, we usually also consume lots of saturated fats, which are actually rather bad for our bodies. When burning fat, we also need to get rid of the toxins of it, that's what water can be good for.
Also, when we're trying to lose weight, most of the weight we actually lose it body fluids. The scales can be a bit tricky in this. It's very important to stay well hydrated when you're trying to lose weight. The numbers on the scale aren't most important, it's what happens within your body.
Of course, taking hot showers, drying out your hair by straightening it, tanning, ... Water alone might not be enough for you to keep your hair and skin well hydrated, but it's a good start to stay hydrated from the inside out.
It's best to drink just water, but what if you really hate drinking it? Try to add some flavour in your water, a few drops of lemon juice or grenadine. Drink juices (with no added sugars is best, as fruits already contain lots of sugars). Also foods that contain a lot of water like some kinds of fruits or soup could be good for you. Try to stay away from coffee or soda.
Also don't drink too much as that could also be dangerous. About 2 litres should be enough for an avarage person.
Take care of your body!