Does the phenomenon of inertia exist by itself as a curious aspect of nature? No! We can see the reason for inertia in terms of energy. We have seen that motion is a form of energy; a moving mass is capable of causing things to go, work, or change. We have also learned that energy is neither created nor destroyed; it is only changed from one form to another.
Therefore, a mass at rest remaining at rest is simply to say that it will not move unless/until it gains energy. It gains energy (movement energy) from a force being applied. Conversely, a moving body remaining in motion is saying that to slow down and stop, its movement energy must be removed. On earth this generally occurs through friction, running into something, or the force of gravity.
Note and emphasize how the phenomenon of inertia reinforces the concept that energy is neither created nor destroyed.