Install Now Avoid Crisis Mode
The project manager will make certain the system has been engineered properly so the right equipment will be ordered and delivered on time. He/she will make sure the right wires have been run to the right places, that equipment cabinetry has adequate space for all the components and is properly ventilated and wire chases provided for (have you ever shown up to do an installation to find the products you sold won't even fit?).
A project manager will greatly improve your installation efficiency by making sure the job site is actually ready for you when they say it is. They also write work orders so field technicians will know what to do and finish punch lists, thus reducing site visits and call-backs.
A project manager will attend job site meetings, coordinate project details, provide product planning (what they need and when it's needed), manpower planning and create punch lists. Most importantly, they are the assurance you need to get the job done efficiently and in a timely manner to ensure a happy client and a paid invoice.