Why does Isaac Newton's first law mainly explain inertia?

Answer 1

The object at rest will remain at rest unless there is an outside force applied.

Example:

There is a Basketball on the floor. Suddenly, a soccer player kicked it.

Explanation:

While the basketball is on the floor, it applies force, that is why it is stable--not moving.

Then when the soccer player kicked the ball, that is the time that an outside force was applied.