Answer :

Xepedosthens
Plants don't need "food" to function. They use sun light/photosynthesis and minerals from the soil to make the sugars they need in order to grow and survive. Animals are not at all like plants (Due to the fact that animals can't use sunlight to produce the energy that they need to live), so they need to take in organic/physical sources of energy in order to function. So that's one reason why animals eat, and plants don't. :P 
Animals must eat because they are a living mammal. Plants are not humans or living mammals, all they need is sunlight, rain. Animals should it because they need their nutrition to live. Plants also need nutrition but not as in eating something, they get it.