To understand the difference between intracellular and extracellular water, please first read:
What is Intracellular Water (ICW) and Extracellular Water (ECW)?
Measuring intracellular and extracellular water gives you a more complete picture of the body's hydration status. Total Body Water does not give you any indication of whether you have a healthy ratio of intra to extracellular water. For optimal health this ratio should be approximately two thirds intracellular and one third extracellular.
You may have a healthy amount of water in your body, however, if you suffer from water retention, this would only be visible when the ratio of intra to extracellular water is being monitored.
From a sports performance monitoring aspect, any reduction in intracellular water can lead to reduced mental and physical function, so the ability to monitor this for individual players and therefore plan individual hydration strategies is crucial.