Your body uses the materials in food

to maintain your body

your organs, for example maintaining your body temperature or the heartbeat needs fuel.

Maintaining body temperature

to go on with activities

like singing, playing football or petting a cat.

Petting a cat

when you are young and to replace dead body cells any time. For example you lose your skin (it peels) and it builds up incessantly.  If you study a lot, you need more material in your brain, too to build new connections.