Because we need to understand how the Earth, our very own planet, works. Knowing its layers is indispensable because we are part of this planet. And it is our right to know why the occurrences of some phenomenons happen such as what causes volcanoes to erupt, why are there high tides and low tides, why are there tsunamis and earthquakes, the gravity, and the likes which can only be answered via understanding the layers of the Earth-the inner and outer core, the mantle and the crust. Only then, we can tell of how great God is.
Moreover, Science pushes us to study it whether we like it or not. That's just it.