Why is the Hess' law important in physics?
First law of thermodynamics
The first law of thermodynamics states that the energy remain constant in a system. You can transfer energy, but you cannot create or destroy it. Or what every chemistry/physics professor loves to say "there's no such thing as a free lunch!"
Hess' Law is just a way of observing the transfer of energy through chemical reactions. It shows mathematically how energy is transferred, but never created or destroyed.