Here's why:

* Cellular respiration is the process by which organisms break down glucose (a sugar) in the presence of oxygen to release energy in the form of ATP (adenosine triphosphate). This energy is then used to power various cellular processes.

* Plants produce glucose through photosynthesis, but they still need to break it down to access the energy it contains. This is why plants also undergo cellular respiration.

* Animals obtain glucose from the food they eat. They rely entirely on cellular respiration to obtain energy from the glucose.

So, while plants have the unique ability to produce glucose through photosynthesis, both plants and animals use cellular respiration as the primary means of generating energy for their cells.