if inertial mass and gravitational mass are exactly the same, everything will fall at the same speed, making it hard for a falling observer to establish that he is in a gravitational field.
Einstein hypothesized that, not only would it be hard, but in fact impossible. This thought lead him down a rabbit hole of curved spacetime and gravity being geometry, and in the process he (arguably) solves the mysteries of action at a distance and inertial/gravitational mass, that plagued Newton. Later investigations of the theory exposes strange effects such as how the center of a black hole is not a place in space but a moment in time.
This is the story of General Relativity.