Question

Why might an animal need one particular macromolecule or nutrient rather than another?

Answer 1

Requirement of macromolecules has evolved through millions of years.

1. It might be possible that when life began that particular macromolecule was very abundant as a result body of the animal adapted to use the abundance of the macromolecule as the defining property to use that.

2. One organism feeds on another. So it's prey might be having that particular macromolecule in its body so the animal had to adapt to assimilate the macromolecule.

3. That particular macromolecule must be have specific oxido-reductive, catalytic property etc required by the animal thus must be needing it.