why are these chemicals being changed into some other biomolecules?? can u plz explain wid examples ??

Dear Student, 

Here they are referring metabolic reactions/ metabolism.

• Metabolism is sum of various biochemical reactions taking place in our body. 
• Metabolic reactions are of two types : catabolism and anabolism. 
• Catabolism: In this type of reaction larger molecules are broken down in smaller molecules. 
• For example:  Breakdown of fat in adipose tissue to fatty acids
• Anabolism: In this type of reaction large  biomolecules are produced by combining smaller biomolecules.
• For example: Synthesis of proteins by polymerisation of amino acids. 

Metabolism is essential for proper functioning of our body hence these biomolecules are getting converted to other form. 

Regards.