Glucose is the main source of energy in cells, being soluble, easily transported, and having a high energy content. Glucose can also be converted to storage molecules, with glycogen being used to reduce blood glucose levels and provide a longer term energy store. Glucose can be converted to fats from acetyl CoA, allowing a higher energy content (by mass) to be stored (although fats are not water soluble, and so are more difficult to mobilise). Fats are also light, enabling animals to be mobile.
Glucose is the starting molecule for glycolysis, with many products of the subsequent pathways (link reaction, Krebs cycle) being used for further metabolic reactions to produce secondary metabolites.