What is the significance of Mass excess(why is it used)?

mass excess is a measure of nuclear bindin energy

dear abhinav,

this is a relative term, mass of the products compared to mass of the reactants. In nuclear reactions, energy is needed to overcome the coulomb repulsions in bringing the nucleons together to form the nucleus ( as the product nucleus is to be formed again, which is different from the one before). Then, who is going to supply this energy? it has to provide itself, luckily for us the explanation is easy using mass-energy relation by which a small equivalent mass is converted to energy in forming the new nucleus and also some energy is released.  hence, a mass difference or mass excess comes into picture.