Question

how does decomposition of dead animal turn out to be useful

Answer 1

After the death of certain animals after few they get turn into fossil fuels which are very useful for the growth of plants.

Answer 2

$\huge{\underline{\underline{\color{darkblue}{\bf{♡Answer♡:-}}}}}$Decomposers breaking down the dead and decaying animals into minerals. These minerals get mix up with the soil and then again used by plants.