why does inertia exists meaning why there is a inherent capacity of a body to remain in a state of rest or motion on its own and oppose the change in its position?
The idea of inertia is quite simple. Inertia is the principle that an object will tend to keep moving at a constant speed and in one constant direction unless something else acts to change it. That external force could be almost anything.
Friction is a common external force that can change the speed and direction of a moving object. Many other forces can also affect motion, however, from gravity of the earth's pull to another person. The key idea is that an object will keep moving or stay still unless something else influences it to do otherwise.
Inertia helps to explain why many objects act in specific ways. For example, understanding the role of external forces in the principle of inertia helps a person understand why a ball that is thrown up in the air doesn't keep flying forever. It also helps to explain why brakes use friction to help slow down a bike, or why an apple falling from a tree is stopped when it hits the ground.