With intense activity, glucose is needed. The participants of the ski runs are given sugar along the way. Why?

Glucose is a monosaccharide that is the main and fastest source of energy.
As you know, energy is necessary for the work of the whole organism, including muscles. When skiing long distances, the energy obtained from food is quickly consumed, then glucose is obtained by splitting glycogen (it is in the muscles, liver, kidneys and brain), and when its reserves are exhausted, there is no energy left for the race. To avoid this, ski runners are given sugar along the way, which is a light source of glucose, and the runners can continue running.