This spontaneous transfer of electrons from zinc to copper is not useful because the free energy released is dissipated as heat.

It is analogous to burning a spoonful of sugar with a match instead of eating it and converting the free energy of oxidation into useful muscle work.

If some means could be found to separate the removal of electrons from zinc (oxidation) from the donation of electrons to copper ions (reduction), then the electrons might be made to do something useful along the way.