A clear vision and roadmap for the team to follow, regardless of whether that vision is owned by a single person or collectively by the whole team.
Technical leadership and better coordination.
A stimulus to talk to people in order to answer questions relating to significant decisions, non-functional requirements, constraints and other cross-cutting concerns.
A framework for identifying and mitigating risk.
Consistency of approach and standards, leading to a well structured codebase.
A set of firm foundations for the rest of the project.
A structure with which to communicate the solution at different levels of abstraction to different audiences.