I have a very simple reasoning about it. It's only based on personal experience, so don't regard it as absolute truth.
Your body needs a certain amount of water to be comfortable, say, N. Comfort here includes urination (removal of poisons from body) and sweating (removal of heat). If you drink N or more, the body will function well.
If you drink less than N, the body will adjust to the deficit of water; it will decrease urination (so unwanted stuff will accumulate in the body), and then reduce sweating (body temperature will rise, leading to bad mood and less-than-optimal physical power).
You can often tolerate these effects (e.g. in emergency; also in general, depending on your goal and attitude), so you should drink less than your "normal" need.
In additon, imagine that you decided to conserve your water and drink 80% of N during the day, and you accept the ill effects of doing so. If you temporarily exceed that plan, drinking e.g. 30% of N in a quarter of a day, the body will "use" the excess water for natural but undesirable functions (urination/sweating). So you should be careful about how much you drink, and not succumb to thirst easily.
The tradeoff between longevity and performance is not easy to resolve; I feel that if you are carrying your water on your back, the tradeoff is always to drink less than needed for optimal performance. Some people will surely disagree though.