Sodium is the primary electrolyte lost in sweat. Additionally, it aids in hydration by maintaining proper water balance. This is such a crucial part of any type of endurance training that it should be a top priority when selecting a nutritional supplement.
When running a challenging race you are going to sweat and when you sweat you lose salt and that salt works towards maintaining a healthy balance for your body so the more you lose the more your balanced body functionality is impacted. Replacing the loss of sodium keeps the body's functionality in check which also means your body is more up to the task of taking on any enduring challenges. If you lose too much sodium and your electrolyte balance gets adversely affected there will be a gradual string of additional complications both physically and mentally.