Why is the ellipsoid important?

Because of their relative simplicity, reference ellipsoids are used as a preferred surface on which geodetic network computations are performed and point coordinates such as latitude, longitude, and elevation are defined.

Why do we need a geoid?

A geoid is the irregular-shaped “ball” that scientists use to more accurately calculate depths of earthquakes, or any other deep object beneath the earth's surface. ... If Earth were a perfect sphere, calculations of depth and distances would be easy because we know the equations for those calculations on a sphere.