Game development is a bit like space exploration – you really shouldn’t do it without making a plan
Before reaching for the stars it helps a great deal to know where you are in the universe. How much fuel do you have? What is the purpose of your journey? What are possible risks and how do you avoid them? Those are just a few questions a project manager will work through with you, so he can prepare accordingly. And though this sounds pretty simple, remember that the near earth orbit is filled with debris of unsuccessful projects. The number of failed game developments is unfathomable higher
Having someone experienced who is painting out a project on paper with your team, tracking your progress, finding individual solutions to problems you couldn’t possibly imagine ahead of time, that is what a project manager is for.
You should never departure without one.