Why is it necessary to learn the other clefs?

If you are a saxophonist, why should you learn the alto clef? I ask because my sister is studying music theory with the saxpohone at the Birmingham conservetoire (junior) and she asked me. I told her that it is important to learn the other clefs to give you a better sense of where yours fits in, but I was really just talking drivel and she knew straight away that I was talking drivel. What can I say to her that is a good answer for learning the other clefs, because besides the ambition to be a composer or a conductor, then I really can't see one!!