Johanna Rothman at Managing Product Development talks about the importance of having a vision for a project. Especially a project with sub-projects, a vision will provide an understanding on how employees’ effort fits into the overall product:
… Even if the PM had started with a vision of the product as a deliverable, it’s not clear the sub-project teams would be aligned. But there’s nothing to bring this program together. I don’t know if my suggestions for the project are reasonable, but without more information, they seem reasonable to me. :-)
People are smart. People want to do a good job. And when sub-project teams move in different directions, it’s because they think that’s the best way to accomplish that team’s work. Without a project (or program) vision, people have no way to make decisions. A vision is the first decision a project manager makes. It’s certainly not the last.
A Project Needs a Vision – [Managing Product Development]