The physics books say energy is MV^2. What is the physical significance of this formula? Why is it an important quantity rather than say, momentum. For example, why do we compare rockets in terms of energy?
The formula (1/2 mv^2) you are asking about is specifically kinetic energy. Trust me, keeping things in terms of conservation of total energy simplifies a lot of situations and makes for easier solutions. This is hardly so with respect to keeping things in terms of conservation of momentum.
The conservation laws are as fundamental and important as the universal constants.