Cause living things can not directly use most of matter which they found in nature. For example carbon, all organism use carbon in complex structure, that is in compound or macromolecule(glucose,fructose,amino acid). we can not directly earn ATP from C which we found in soil/other sources.
When a phosphate group is removed from the molecule
1. natural selection acts on existing traits because you can not evolve into something you didn't have
2. original structures evolve to accomplish new function in order to survive