Location:
You might be lucky enough to live in close proximity to your yacht, but lets face it, most owners do not. If you live and work far away from the yard handling your project, any issues that require immediate attention, such as costing, emergent works and overall quality control, will be very difficult. Again, investing in a PM will save you a lot of time and money as he or she would handle any of the above.
Experience:
Do you have experience in project management, boatbuilding, costings, industry best practice and a mass of connections? If the answer is yes, excellent! If not, with the right project manager, you will gain that experience, get someone to react on your behalf at a moment's notice and someone who will always "fight your corner".
Money:
This is perhaps the most significant aspect of a project. An experienced project manager can accurately assess projected costs and will negotiate on your behalf to ensure you get the best value for money, whilst maintaining the level of quality you desire. And here´s a hint; as your PM will very likely know the boat yard involved in your project, he can keep a close eye on them and ensure they deliver on time.