Here's why:

* Source of Carbon: Plants use carbon dioxide as the source of carbon atoms to build sugars, the primary product of photosynthesis.

* Energy Storage: These sugars store chemical energy from sunlight, which the plant can then use for growth and other functions.

While water (H2O) is also essential for photosynthesis, it is not the compound that provides the carbon. Water acts as a source of electrons and hydrogen ions, which are used to convert carbon dioxide into sugar.