Here's why:

* Cellular Respiration: Glucose is broken down through cellular respiration, a process that releases energy in the form of ATP (adenosine triphosphate). ATP is the primary energy currency of cells.

* Food Sources: Glucose is readily available from food sources like carbohydrates (sugars, starches), and even proteins and fats can be converted into glucose.

* Photosynthesis: Plants and other photosynthetic organisms use sunlight to convert carbon dioxide and water into glucose, which they then use for energy and building materials.

While other molecules like fats and proteins can also provide energy, glucose is the most readily accessible and commonly used source for most living things.