For example - would you want to be a pianist that reads off sheet music all the time, or one who actually knows how to play songs from their head and improvise?
Actually, scratch that. Tracing is like playing sheet music with the recording of that song going off in the background for comparison. Sketching from reference or a life source is like playing sheet music. Illustrating, which I imagine is what you'd want to do, is improvising. Now there are skills that are common to all three forms, but the best way to get good at something is to practice doing that very thing.
Tracing is like playing music with the letters written under the notes and on the keys.