Here's why:

* Energy Source: Glucose is a simple sugar that serves as the primary energy source for plants. It provides the energy needed for all their vital processes, including growth, reproduction, and repair.

* Building Block: Glucose is also a building block for other important molecules, such as cellulose (the structural component of plant cell walls) and starch (a storage form of energy).

While photosynthesis produces oxygen as a byproduct, it's not the primary product that sustains plant life. Oxygen is a waste product for plants, though it's crucial for the survival of most other life forms on Earth.