What the model defines
A governance model specifies the architecture of deciding: the forums (board, executive committee, investment committee, steering groups), the decision rights each holds, the thresholds that route a matter to one forum rather than another, escalation paths, meeting cadence, and the information each body is entitled to see. It exists at corporate level and, in miniature, inside every major programme.
Why structure shapes outcomes
Decision quality is usually discussed as a property of people, but structure decides what those people get to work with: whether challenge happens before or after commitment, whether the forum reviewing a proposal is independent of the team advocating it, whether anyone owns the outcome once the meeting ends. A sound model places each decision at the level holding the information and the accountability. A poor one routes everything upward until executives are approving choices they lack context for, at a pace that guarantees the approvals are ceremonial. When a major commitment is being prepared, the governance question is as material as the commercial one: who decides, on what evidence, against whose challenge?