Why so earth scientists use 2 different ways of classifying earths layers ?

The Earth’s layers can be categorized mechanically or chemically. The chemical classification is more simple and includes the inner and outer core and the mantel which is divided into the upper and lower mantle. The crust is the surface. However, in the mechanical classification divides the lithosphere, the asthenosphere and the mesospheric. It is important to have the mechanical classification to understand the forces that move the tectonic plate.