[vannevar]

>Every professional understands priority and expectations. And communication is all that is required.

Sure--the kind of communication that is ensured by a brief daily standup (communication within the team) and a regular progress review with the stakeholders, like say, a sprint review every few weeks (external communication).

While it's true that every professional understands the idea of priority, the global priorities of a project may be very different from the individual priorities of a developer. A diligent, conscientious developer can still get caught up in a narrow problem that doesn't really move the project forward, even after the PM has "made it clear what should be worked on." Fire-and-forget is not a strategy for project success. Continuous communication and regular reviews/resets are a better way.