There are so many conflicting reports out there that say either you have to drink 8 glasses of water or you don't. And each is more vociferous than the other.
There are times when I forget to drink water. Odd but true. But I see the effects on my skin which becomes dull. Besides, your mouth becomes extremely dry. So now I have countered this by keeping a glass of water by my desk always and I keep sipping water throughout the day.
I would say how much water to drink is up to you. Some people would require more than 8 glasses of water according to their level of physical activity. But trying to achieve 8 glasses as a benchmark is good for the rest of us. One good reason to drink water- to prevent kidney stones. Dehydration increases the chance of having more kidney stones.
Drinking water also keeps you full so that you don't end up eating more. Try drinking a glass of water before your meal.
Dehydration also causes fatigue, decreased blood pressure,constipation and even unconsciousness