Question

why is glucose given to sportsmen ?

Answer 1

Glucose is a form of simple sugars/carbohydrates which are present in the blood of all kinds of animals including Humans. Glucose doses should be maintained in the flood or optimal levels of energy and alertness. 
Sportsmen need to be active and maintain high levels of energy,  a simple sugar like Glucose, breaks down easily in the flood providing quick and efficient energy dosage to the sportsman to help them perform well.
For similar reasons, some sports people rely on eating bananas' as they also digest quickly and provide a short natural burst of energy