Why is law of conservation of momentum important?
The law of conservation of momentum says that from every interaction, the total momentum will be the same before and after. Momentum, like velocity is a vector quantity, so its direction matters. Momentum is mass multiplied by velocity.
In the case of an inelastic collision momentum is conserved. Kinetic energy however is not conserved. The reason being that some or all of the kinetic energy gets converted to heat energy.