The units someone uses are ultimately going to depend on what they need them for. Someone working sending interplanetary radio signals might find light-minutes most useful, while someone studying orbits may likely use AU’s. If you’re trying to describe the size of orbits, using the orbit of Earth as a reference is really useful because it’s a distance that’s easy to compare to. It’s the same reason you don’t describe your weight in tones, differences of a few pounds are hard to conceptualize using units that are many times larger/smaller than what you can conceive.