There are many scientific reasons why creatures have to eat. I personally enjoy a good meal, unfortunately I eat the types of food which scientists warn are bad for us and that the food companies entice us with on the television, radio and in the newspapers. As humans we are constantly being supplied by this energy day in and day out, keeping our bodies refreshed and maintained. I believe that this is why we eat. The two main groups that we can group the food we eat into are plant based and animal based.
Lets start with animals.
When an animal dies the body starts to decay, this is because there is no longer a need to replenish the animal when it is dead. What is left over, however, is a huge storehouse of concentrated aether units slowly breaking down. Plants can sustain us just as well as animal based foods can, only the frequencies required are spread over a greater variety of fruit, vegitables, nuts etc. There have been stories of animals like cats and dogs, natural carnivours, spending great lengths of time inside a pyramid, then after a period of time starting to prefer vegitation to meat.