The purpose of DoL isn't to enhance battle classes it is to provide materials for crafters.
Providing materials, turning materials into items, all exist for one purpose: enhancing battle classes.

None of these classes is in the game for the sole purpose of making another class better
Yeah, except DoL and DoH.

DoW and DoM do nothing to help you. They give you money, thats their part in this "harmony". If not DoM and DoW, you exist to make your Company's houses better or your ships more robust. Your whole point in this game is to support everyone else.

There is no other justification for their existence, not in a fantasy world nor in real life. That's what you do! That's what you've been created to do. If you don't find it engaging, then goddamn it son. Don't do it.