A project manager is different from an interior designer in that the project manager oversees your project: from beginning to end, whether designing or building. So how do you know if you really need one and what it might cost?
A project manager will oversee the schedule of your project and make sure it is all on track. They will coordinate with your builder, architect, designer, and tradesmen to make sure everything is on track and on budget.
Make sure that you ask for references and check them out. Architects and builders are most familiar with project managers, though some designers might have contacts. Expect to meet with them on a regular basis, and make sure that you both understand those parameters and expectations.
A project manager may charge a flat fee, or they might charge a percentage of the job. They are not as expensive as the architect or builder, and are generally less than the designer.
Is it worth it? Absolutetly, for certain projects. For people moving from another country or relocating across the country, it is an invaluable service to know that someone is checking on your job. For people who are busy traveling with two careers and juggling family, it is worth it. And for jobs that are very large and very expensive, it is definitely worth it. I suggest that anyone considering a home renovation project hire a project manager. Even though it might cost a little bit, a project manager will be an independent and objective gatekeeper with your builder and designer.
As mentioned: check references. Know your parameters and needs, and make sure those are clear on the front side. Then you can relax knowing that you are in the hands of professionals, and have a professional managing those professionals. Good Luck!