# Precession - why decompose to angular momentum component?

I always see it is conventional (in Landau-Lifshitz for example) to decompose the angular velocity vector to 2 components: axis of symmetry and angular momentum vector, and then define the angular momentum part as $$\Omega_p$$, meaning the precession component. My question is, why? Why not orthogonal components? Why is the angular momentum component considered the precession component? Is it a mere definition?