Inhibitor

You need to read a biochemistry textbook to get proper answers to these questions. Short version: the oxidation of a substrate and subsequent transfer of electrons through the electron transport chain results in a pumping of protons outward across the inner mitochondrial membrane. This electrochemical proton gradients represents a pool of potential energy which is then used to synthesize ATP from ADP + Pi by ATP synthase.

Basically this. You question may be better answered by a philosopher or something, but in the realm of science, things may just not have a "purpose" as we understand it in everyday terms. The best answer I can give would be that things that are thermodynamically favorable just happen.