When you exercise for more than three hours, you should take in salt as well as fluids. A study from Switzerland followed female competitive distance runners who took in drinks with different concentration of salt during a four hour run. Ninety-two percent of those who took in plain water with no additional salt developed low blood levels of salt, which can be dangerous (British Journal of Sports Medicine, Volume 37, Issue 4, 2003).
Taking in fluid without also taking in adequate amounts of salt dilutes the bloodstream, so that the concentration of salt in the blood is lower than that in brain cells. This causes fluid to move from the low-salt blood into the higher-salt brain causing the brain to swell which can cause seizures and death. Taking in extra salt during prolonged exercise increases thirst so you drink more fluids, and prevents blood salt levels from dropping so low that you become tired, develop muscle cramps, and can even die. Furthermore, without salt you do not recover as quickly and are more likely to be injured or tired all the time.
If you’re concerned about the reports of deaths from over-hydration, read about hyponatremia