What conclusion can be drawn from the law of conservation of total mechanical energy?

Energy does not disappear anywhere without a trace. It only transforms from one type to another.
For example, a metal body was suspended at any height – it had potential energy. If we set it in motion (cut off the thread), then it will have both potential and kinetic energy. At the moment when the body is about to come into contact with the ground, all potential energy will go into kinetic energy, and that, in turn, into internal energy. This is the basis of the principle of conservation of energy.