It's because the earth isn't round - if it was, we would just use geocentric. Instead we have to draw a tangent at the horizon, then a line 90 degs to that, and the angle where it meets the horizontal at the centre of the Earth is the geodetic. The difference for our purposes is negligible, and if the earth were the size of a billiard ball you wouldn't notice any difference between it and the others on the table.
I don't see much point to it (for the exams at least), except maybe to say that the mathematical model inside the GPS (based on the World Geodetic Standard established in 1984) is different enough for you to be careful if your track takes you near to the boundaries of airspace or danger areas.