Inertia is directly related to mass of an object. Law of inertia, says that a body continues to be in the state of rest or uniform motion, unless an external force acts on it. We are talking about the inertial property of objects.
This law gives us predictability of state of bodies. Otherwise, just imagine, you put your belongings on a table and they move away (if there is no inertia). We can only calculate various physical quantities needed to stop a moving object, or to move a stationary object, by assuming inertia of objects.
Constructions where high stability is needed, structures and shapes of high moment of inertia are used. Like the I beam under the ceiling, and the I shape of the rails on a railway track.
If our brain is not able to retain the knowledge we study on a day, then what happens? We have to read again and again.If objects did not continue in their state, then every instant we have to act on them to maintain their state.
Inertia here is similar to continuing to remember what we have studied.