Purpose is the architectonic construct that defines why a unit exists, whom it serves, and under what conditions its presence is justified. It operates as the primary structural constraint, shaping Function, Process, Structure, and all subsequent design decisions. Without Purpose, sociotechnical systems drift into incoherence, losing both internal alignment and external legitimacy. This chapter establishes Purpose as the first principle of unit design—defining its properties, its role in stabilizing internal architecture, and its function in ensuring systemic and environmental fit.