This means that we are less suseptible to dehydration and thus can exercise for longer or at a higher intensity. Sodium improves palatibility as it offsets the sweetness of the added sugars. There is also some evidence that after exercise and when dehydrated most people have a preference for a slightly salty tasting beverage Sodium and other electrolytes do need replacing in ultra endurance events over 4 hours. The most common problem arises in those who sweat profusely, as they are at risk of both dehydration and sodium depletion. Many athletes don't drink enough on a day to day basis and often go into events already partially dehydrated.
Fluid is absorbed faster if small amounts are drunk regularly.
If already dehydrated, fluid will be absorbed slowly and may sit in your stomach or cause a stitch during running. Dehydration can also cause gastric upset, so runner's diarrhoea can be caused by lack of fluid as much as by the food you eat beforehand. Dehydration can also affect mental functioning and co-ordination. Start by drinking regularily throughout the day. Don't save up until training.