The concept of drinking 8 glasses of water forcibly is not based on any logic at all. It is natural for the body to demand water when it feels the need for it. It is just like hunger for food when the sugar levels start dipping in your body.
Your body requires water for the purposes below
Water which is lost through the kidneys, bowels and your skin would require to be compensated for. This is when you feel thirsty. A glass of water should usually take care of this demand.
Human being have 60% fluid in their body. It is essential for their survival to maintain the fluid quantity by replenishing the lost water.
Blood plasma contains close to 92% water and is essential to conduct nutrients to different parts of the body. Water is important to dissolve harmful gases and expel it through lungs. Water regulates the body temperature to ensure there is not even a minor fluctuation.
The most natural source of water for human being is through their food. Unfortunately the modern day food consisting mostly of processed and cooked items hardly contain any water content at all. A diet rich in meats and other non-vegetarian items, alcoholic beverages and soft drinks, leaves the body starved for water.
The purest source of water is obtained from fruits. People who partake of a diet rich in fruits hardly ever feel the need to compensate their water needs through drinking.