Your
body tissues are made primarily of water - I believe the figure is close to 70%. Your blood, spinal fluid, the mucus membranes in your lungs, the fluid that pads your joints, the fluid that fills every cell in your
body - all of those things are a delicate balance of water and other chemical elements.
When you drink (and eat, because there's water value in food as well) too little water, it upsets the "balance" within your
body.
When that balance is upset, your
body works to restore the balance. One way it restores that balance is by retaining water/fluid that would otherwise be excreted (either by peeing, by sweating, or just in respiration).
By the same token, when you eat too much sodium, it causes an imbalance and so your
body retains more water to "dilute" the sodium and minerals that you've consumed. That's why when you have a sodium heavy meal (or day), it's a good idea to drink a little more water to help restore the proper mineral balance in your system and keep you from retaining fluids.
Additionally, keeping water flowing through your system - through your kidneys and bladder - helps flush out the impurities in your system more efficiently.
So drinking enough water to keep your system balanced and flushed is hugely important. The whole "8 cups a day" thing is a
myth that has been busted - just like 1200
calories to lose weight. 8 cups a day doesn't take into account individual weight, levels of exertion, etc. The best guideline I've seen were put out by the International
Sports Medicine Institute. They start with 1/2 ounce per
pound of
body weight for someone who is inactive, and go up from there as you increase activity.
So for example, my weight is 170, half of that is 85. So they recommend 85oz of water a day (which works out to about 10.5 cups). I drink about half again that much - but then I also work out 4-5 times a week.