You're absolutely correct, the purple health bar is tied directly to the bosses final health. So while it's not completely required to clean all the nodes, it makes the final boss an easier fight. It is supposed to be more of a hidden mechanic you wouldn't know about until the end of the game, but in hindsight more hints probably would have been for the best. Thank you for the feedback, and for being so perceptive to our programmer's hard work!