Here's why:

* Light: Plants use sunlight for photosynthesis. This process converts light energy into chemical energy, which is stored in sugars. This energy fuels the plant's growth and development.

* Heat: The sun provides warmth, which is essential for plant processes like germination, growth, and flowering. Plants need a certain temperature range to thrive.

While there are some plants that can survive in low light conditions or even grow in artificial light, the sun is the primary source of heat and light for the vast majority of plants.