The foremost reason as to why you need double glazing is that double glazed windows are purposely designed to reduce heat loss from homes, by capturing and storing a higher percentage of natural heat from the sun, acting as a form of insulation. 

However, the double benefit of this technology is that during the summer months, double glazing prevents your home overheating, by trapping sun rays and minimizing the heat which burns through single-pane windows during hot, summer days.