Law of thermodynamics states that matter and energy cannot be destroyed or created, but can be converted from one form to another. A balanced equation ensures that this basic principle is observed.
On the left-hand side of the equation (the reactants), there are 2 Nitrogen moles and 8 Hydrogen moles. After the reaction, the molecules are rearranged. Nonetheless, the moles of Nitrogen and Hydrogen in the products remain the same.